diff --git a/CHANGELOG b/CHANGELOG
index a6cd8f4c7e1..cc98863dac8 100644
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -223,6 +223,7 @@ v 8.10.6
   - Restore "Largest repository" sort option on Admin > Projects page. !5797
   - Fix privilege escalation via project export.
   - Require administrator privileges to perform a project import.
+  - User can edit closed MR with deleted fork (Katarzyna Kobierska Ula Budziszewska) !5496
 
 v 8.10.5
   - Add a data migration to fix some missing timestamps in the members table. !5670
diff --git a/app/helpers/merge_requests_helper.rb b/app/helpers/merge_requests_helper.rb
index db6e731c744..a9e175c3f5c 100644
--- a/app/helpers/merge_requests_helper.rb
+++ b/app/helpers/merge_requests_helper.rb
@@ -98,6 +98,6 @@ module MergeRequestsHelper
   end
 
   def merge_request_button_visibility(merge_request, closed)
-    return 'hidden' if merge_request.closed? == closed || (merge_request.merged? == closed && !merge_request.closed?)
+    return 'hidden' if merge_request.closed? == closed || (merge_request.merged? == closed && !merge_request.closed?) || merge_request.closed_without_fork?
   end
 end
diff --git a/app/models/merge_request.rb b/app/models/merge_request.rb
index 1d05e4a85d1..b41a1f0c547 100644
--- a/app/models/merge_request.rb
+++ b/app/models/merge_request.rb
@@ -91,13 +91,13 @@ class MergeRequest < ActiveRecord::Base
     end
   end
 
-  validates :source_project, presence: true, unless: [:allow_broken, :importing?]
+  validates :source_project, presence: true, unless: [:allow_broken, :importing?, :closed_without_fork?]
   validates :source_branch, presence: true
   validates :target_project, presence: true
   validates :target_branch, presence: true
   validates :merge_user, presence: true, if: :merge_when_build_succeeds?
-  validate :validate_branches, unless: [:allow_broken, :importing?]
-  validate :validate_fork
+  validate :validate_branches, unless: [:allow_broken, :importing?, :closed_without_fork?]
+  validate :validate_fork, unless: :closed_without_fork?
 
   scope :by_branch, ->(branch_name) { where("(source_branch LIKE :branch) OR (target_branch LIKE :branch)", branch: branch_name) }
   scope :cared, ->(user) { where('assignee_id = :user OR author_id = :user', user: user.id) }
@@ -305,19 +305,22 @@ class MergeRequest < ActiveRecord::Base
 
   def validate_fork
     return true unless target_project && source_project
+    return true if target_project == source_project
+    return true unless fork_missing?
 
-    if target_project == source_project
-      true
-    else
-      # If source and target projects are different
-      # we should check if source project is actually a fork of target project
-      if source_project.forked_from?(target_project)
-        true
-      else
-        errors.add :validate_fork,
-                   'Source project is not a fork of target project'
-      end
-    end
+    errors.add :validate_fork,
+               'Source project is not a fork of target project'
+  end
+
+  def closed_without_fork?
+    closed? && fork_missing?
+  end
+
+  def fork_missing?
+    return false unless for_fork?
+    return true unless source_project
+
+    !source_project.forked_from?(target_project)
   end
 
   def ensure_merge_request_diff
diff --git a/app/services/merge_requests/update_service.rb b/app/services/merge_requests/update_service.rb
index 30c5f24988c..398ec47f0ea 100644
--- a/app/services/merge_requests/update_service.rb
+++ b/app/services/merge_requests/update_service.rb
@@ -11,6 +11,10 @@ module MergeRequests
       params.except!(:target_project_id)
       params.except!(:source_branch)
 
+      if merge_request.closed_without_fork?
+        params.except!(:target_branch, :force_remove_source_branch)
+      end
+
       merge_request.merge_params['force_remove_source_branch'] = params.delete(:force_remove_source_branch)
 
       update(merge_request)
diff --git a/app/views/projects/merge_requests/show/_mr_title.html.haml b/app/views/projects/merge_requests/show/_mr_title.html.haml
index 098ce19da21..48016645019 100644
--- a/app/views/projects/merge_requests/show/_mr_title.html.haml
+++ b/app/views/projects/merge_requests/show/_mr_title.html.haml
@@ -1,3 +1,7 @@
+- if @merge_request.closed_without_fork?
+  .alert.alert-danger
+    %p Source project is not a fork of the target project
+
 .clearfix.detail-page-header
   .issuable-header
     .issuable-status-box.status-box{ class: status_box_class(@merge_request) }
diff --git a/app/views/shared/issuable/_form.html.haml b/app/views/shared/issuable/_form.html.haml
index 22594b46443..75753a6b0af 100644
--- a/app/views/shared/issuable/_form.html.haml
+++ b/app/views/shared/issuable/_form.html.haml
@@ -135,28 +135,29 @@
         = icon('question-circle')
 
 - if issuable.is_a?(MergeRequest)
-  %hr
-  - if @merge_request.new_record?
+  - unless @merge_request.closed_without_fork?
+    %hr
+    - if @merge_request.new_record?
+      .form-group
+        = f.label :source_branch, class: 'control-label'
+        .col-sm-10
+          .issuable-form-select-holder
+            = f.select(:source_branch, [@merge_request.source_branch], { }, { class: 'source_branch select2 span2', disabled: true })
     .form-group
-      = f.label :source_branch, class: 'control-label'
+      = f.label :target_branch, class: 'control-label'
       .col-sm-10
         .issuable-form-select-holder
-          = f.select(:source_branch, [@merge_request.source_branch], { }, { class: 'source_branch select2 span2', disabled: true })
-  .form-group
-    = f.label :target_branch, class: 'control-label'
-    .col-sm-10
-      .issuable-form-select-holder
-        = f.select(:target_branch, @merge_request.target_branches, { include_blank: true }, { class: 'target_branch select2 span2', disabled: @merge_request.new_record?, data: {placeholder: "Select branch"} })
-      - if @merge_request.new_record?
-        &nbsp;
-        = link_to 'Change branches', mr_change_branches_path(@merge_request)
-  - if @merge_request.can_remove_source_branch?(current_user)
-    .form-group
-      .col-sm-10.col-sm-offset-2
-        .checkbox
-          = label_tag 'merge_request[force_remove_source_branch]' do
-            = check_box_tag 'merge_request[force_remove_source_branch]', '1', @merge_request.force_remove_source_branch?
-            Remove source branch when merge request is accepted.
+          = f.select(:target_branch, @merge_request.target_branches, { include_blank: true }, { class: 'target_branch select2 span2', disabled: @merge_request.new_record?, data: {placeholder: "Select branch"} })
+        - if @merge_request.new_record?
+          &nbsp;
+          = link_to 'Change branches', mr_change_branches_path(@merge_request)
+    - if @merge_request.can_remove_source_branch?(current_user)
+      .form-group
+        .col-sm-10.col-sm-offset-2
+          .checkbox
+            = label_tag 'merge_request[force_remove_source_branch]' do
+              = check_box_tag 'merge_request[force_remove_source_branch]', '1', @merge_request.force_remove_source_branch?
+              Remove source branch when merge request is accepted.
 
 - is_footer = !(issuable.is_a?(MergeRequest) && issuable.new_record?)
 .row-content-block{class: (is_footer ? "footer-block" : "middle-block")}
@@ -175,7 +176,7 @@
     = link_to 'Cancel', polymorphic_path([@project.namespace.becomes(Namespace), @project, issuable.class]), class: 'btn btn-cancel'
   - else
     .pull-right
-      - if current_user.can?(:"destroy_#{issuable.to_ability_name}", @project)
+      - if can?(current_user, :"destroy_#{issuable.to_ability_name}", @project)
         = link_to 'Delete', polymorphic_path([@project.namespace.becomes(Namespace), @project, issuable]), data: { confirm: "#{issuable.class.name.titleize} will be removed! Are you sure?" },
                                                                                                   method: :delete, class: 'btn btn-danger btn-grouped'
       = link_to 'Cancel', polymorphic_path([@project.namespace.becomes(Namespace), @project, issuable]), class: 'btn btn-grouped btn-cancel'
diff --git a/spec/controllers/projects/merge_requests_controller_spec.rb b/spec/controllers/projects/merge_requests_controller_spec.rb
index c64c2b075c5..f95c3fc771b 100644
--- a/spec/controllers/projects/merge_requests_controller_spec.rb
+++ b/spec/controllers/projects/merge_requests_controller_spec.rb
@@ -170,6 +170,35 @@ describe Projects::MergeRequestsController do
         expect(response).to redirect_to([merge_request.target_project.namespace.becomes(Namespace), merge_request.target_project, merge_request])
         expect(merge_request.reload.closed?).to be_truthy
       end
+
+      it 'allow to edit closed MR' do
+        merge_request.close!
+
+        put :update,
+            namespace_id: project.namespace.path,
+            project_id: project.path,
+            id: merge_request.iid,
+            merge_request: {
+              title: 'New title'
+            }
+
+        expect(response).to redirect_to([merge_request.target_project.namespace.becomes(Namespace), merge_request.target_project, merge_request])
+        expect(merge_request.reload.title).to eq 'New title'
+      end
+
+      it 'does not allow to update target branch closed MR' do
+        merge_request.close!
+
+        put :update,
+            namespace_id: project.namespace.path,
+            project_id: project.path,
+            id: merge_request.iid,
+            merge_request: {
+              target_branch: 'new_branch'
+            }
+
+        expect { merge_request.reload.target_branch }.not_to change { merge_request.target_branch }
+      end
     end
   end
 
diff --git a/spec/models/merge_request_spec.rb b/spec/models/merge_request_spec.rb
index d67f71bbb9c..5fea6adf329 100644
--- a/spec/models/merge_request_spec.rb
+++ b/spec/models/merge_request_spec.rb
@@ -962,4 +962,68 @@ describe MergeRequest, models: true do
       expect(merge_request.conflicts_can_be_resolved_in_ui?).to be_truthy
     end
   end
+
+  describe "#fork_missing?" do
+    let(:project)      { create(:project) }
+    let(:fork_project) { create(:project, forked_from_project: project) }
+    let(:user)         { create(:user) }
+    let(:unlink_project) { Projects::UnlinkForkService.new(fork_project, user) }
+
+    context "when fork exists" do
+      let(:merge_request) do
+        create(:merge_request,
+          source_project: fork_project,
+          target_project: project)
+      end
+
+      it { expect(merge_request.fork_missing?).to be_falsey }
+    end
+
+    context "when source project is the target project" do
+      let(:merge_request) { create(:merge_request, source_project: project) }
+
+      it { expect(merge_request.fork_missing?).to be_falsey }
+    end
+
+    context "when fork does not exist" do
+      let(:merge_request) do
+        create(:merge_request,
+          source_project: fork_project,
+          target_project: project)
+      end
+
+      it do
+        unlink_project.execute
+        merge_request.reload
+
+        expect(merge_request.fork_missing?).to be_truthy
+      end
+    end
+  end
+
+  describe "#closed_without_fork?" do
+    let(:project)      { create(:project) }
+    let(:fork_project) { create(:project, forked_from_project: project) }
+    let(:user)         { create(:user) }
+    let(:unlink_project) { Projects::UnlinkForkService.new(fork_project, user) }
+
+    context "closed MR" do
+      let(:closed_merge_request) do
+        create(:closed_merge_request,
+          source_project: fork_project,
+          target_project: project)
+      end
+
+      it "has a fork" do
+        expect(closed_merge_request.closed_without_fork?).to be_falsey
+      end
+
+      it "does not have a fork" do
+        unlink_project.execute
+        closed_merge_request.reload
+
+        expect(closed_merge_request.closed_without_fork?).to be_truthy
+      end
+    end
+  end
 end
diff --git a/spec/views/projects/merge_requests/edit.html.haml_spec.rb b/spec/views/projects/merge_requests/edit.html.haml_spec.rb
new file mode 100644
index 00000000000..d7a1a2447ea
--- /dev/null
+++ b/spec/views/projects/merge_requests/edit.html.haml_spec.rb
@@ -0,0 +1,41 @@
+require 'spec_helper'
+
+describe 'projects/merge_requests/edit.html.haml' do
+  include Devise::TestHelpers
+
+  let(:user) { create(:user) }
+  let(:project) { create(:project) }
+  let(:fork_project) { create(:project, forked_from_project: project) }
+  let(:closed_merge_request) do
+    create(:closed_merge_request,
+      source_project: fork_project,
+      target_project: project,
+      author: user)
+  end
+  let(:unlink_project) { Projects::UnlinkForkService.new(fork_project, user) }
+
+  before do
+    assign(:project, project)
+    assign(:merge_request, closed_merge_request)
+
+    allow(view).to receive(:can?).and_return(true)
+    allow(view).to receive(:current_user).and_return(User.find(closed_merge_request.author_id))
+  end
+
+  context 'when closed MR without fork' do
+    it "shows editable fields" do
+      unlink_project.execute
+      closed_merge_request.reload
+      render
+
+      expect(rendered).to have_field('merge_request[title]')
+      expect(rendered).to have_css('label', text: "Title")
+      expect(rendered).to have_field('merge_request[description]')
+      expect(rendered).to have_css('label', text: "Description")
+      expect(rendered).to have_css('label', text: "Assignee")
+      expect(rendered).to have_css('label', text: "Milestone")
+      expect(rendered).to have_css('label', text: "Labels")
+      expect(rendered).not_to have_css('label', text: "Target branch")
+    end
+  end
+end
diff --git a/spec/views/projects/merge_requests/show.html.haml_spec.rb b/spec/views/projects/merge_requests/show.html.haml_spec.rb
new file mode 100644
index 00000000000..ed12b730eeb
--- /dev/null
+++ b/spec/views/projects/merge_requests/show.html.haml_spec.rb
@@ -0,0 +1,40 @@
+require 'spec_helper'
+
+describe 'projects/merge_requests/show.html.haml' do
+  include Devise::TestHelpers
+
+  let(:project) { create(:project) }
+  let(:fork_project) { create(:project, forked_from_project: project) }
+  let(:merge_request) do
+    create(:merge_request,
+      source_project: fork_project,
+      source_branch: 'add-submodule-version-bump',
+      target_branch: 'master', target_project: project)
+  end
+
+  before do
+    assign(:project, project)
+    assign(:merge_request, merge_request)
+    assign(:commits_count, 0)
+
+    merge_request.close!
+    allow(view).to receive(:can?).and_return(true)
+  end
+
+  context 'closed MR' do
+    it 'shows Reopen button' do
+      render
+
+      expect(rendered).to have_css('a', visible: true, text: 'Reopen')
+      expect(rendered).to have_css('a', visible: false, text: 'Close')
+    end
+
+    it 'does not show Reopen button without fork' do
+      fork_project.destroy
+      render
+
+      expect(rendered).to have_css('a', visible: false, text: 'Reopen')
+      expect(rendered).to have_css('a', visible: false, text: 'Close')
+    end
+  end
+end