diff --git a/app/models/merge_request_organisation.rb b/app/models/merge_request_organisation.rb
index 6dda8b35e..5bfbe14d7 100644
--- a/app/models/merge_request_organisation.rb
+++ b/app/models/merge_request_organisation.rb
@@ -29,5 +29,12 @@ private
     if merging_organisation_id.blank? || !Organisation.where(id: merging_organisation_id).exists?
       merge_request.errors.add(:merging_organisation, I18n.t("validations.merge_request.organisation_not_selected"))
     end
+
+    existing_merges = MergeRequestOrganisation.with_merging_organisation(merging_organisation)
+    if existing_merges.count.positive?
+      existing_merge_request = existing_merges.first.merge_request
+      errors.add(:merging_organisation, I18n.t("validations.merge_request.organisation_part_of_another_merge"))
+      merge_request.errors.add(:merging_organisation, I18n.t("validations.merge_request.organisation_part_of_another_incomplete_merge", organisation: merging_organisation.name, absorbing_organisation: existing_merge_request.absorbing_organisation&.name, merge_date: existing_merge_request.merge_date&.to_fs(:govuk_date)))
+    end
   end
 end
diff --git a/config/locales/en.yml b/config/locales/en.yml
index 18fc6d2c4..698618717 100644
--- a/config/locales/en.yml
+++ b/config/locales/en.yml
@@ -186,6 +186,8 @@ en:
               more_than_year_from_today: "The merge date must not be later than a year from today’s date."
             existing_absorbing_organisation:
               blank: "You must answer absorbing organisation already active?"
+            merging_organisation_id:
+              part_of_another_merge: "Another merge request records %{organisation} as merging into %{absorbing_organisation} on %{merge_date}. Select another organisation or remove this organisation from the other merge request."
         notification:
           attributes:
             title:
@@ -371,6 +373,7 @@ en:
         during_deactivated_period: "The location is already deactivated during this date, please enter a different date."
     merge_request:
       organisation_part_of_another_merge: "This organisation is part of another merge - select a different one."
+      organisation_part_of_another_incomplete_merge: "Another merge request records %{organisation} as merging into %{absorbing_organisation} on %{merge_date}. Select another organisation or remove this organisation from the other merge request."
       organisation_not_selected: "Select an organisation from the search list."
 
   soft_validations:
diff --git a/spec/requests/merge_requests_controller_spec.rb b/spec/requests/merge_requests_controller_spec.rb
index 869b73568..a73db8067 100644
--- a/spec/requests/merge_requests_controller_spec.rb
+++ b/spec/requests/merge_requests_controller_spec.rb
@@ -84,6 +84,22 @@ RSpec.describe MergeRequestsController, type: :request do
         end
       end
 
+      context "when the user updates merge request with organisation that is already part of another merge" do
+        let(:another_organisation) { create(:organisation) }
+        let(:other_merge_request) { create(:merge_request, merge_date: Time.zone.local(2022, 5, 4)) }
+        let(:params) { { merge_request: { merging_organisation: another_organisation.id, new_merging_org_ids: [] } } }
+
+        before do
+          MergeRequestOrganisation.create!(merge_request_id: other_merge_request.id, merging_organisation_id: another_organisation.id)
+          patch "/merge-request/#{merge_request.id}/merging-organisations", headers:, params:
+        end
+
+        it "displays the page with an error message" do
+          expect(response).to have_http_status(:unprocessable_entity)
+          expect(page).to have_content("Another merge request records #{another_organisation.name} as merging into #{other_merge_request.absorbing_organisation&.name} on 4 May 2022. Select another organisation or remove this organisation from the other merge request.")
+        end
+      end
+
       context "when the user selects an organisation that is a part of another merge" do
         let(:another_organisation) { create(:organisation) }
         let(:params) { { merge_request: { merging_organisation: another_organisation.id, new_merging_org_ids: [] } } }
@@ -400,7 +416,7 @@ RSpec.describe MergeRequestsController, type: :request do
         context "when merge date set to a date more than 1 year in the future" do
           let(:merge_request) { MergeRequest.create!(requesting_organisation: organisation) }
           let(:params) do
-            { merge_request: { page: "merge_date", "merge_date(3i)": "#{Time.zone.now.day + 1}", "merge_date(2i)": "#{Time.zone.now.month}", "merge_date(1i)": "#{Time.zone.now.year + 1}" } }
+            { merge_request: { page: "merge_date", "merge_date(3i)": (Time.zone.now.day + 1).to_s, "merge_date(2i)": Time.zone.now.month.to_s, "merge_date(1i)": (Time.zone.now.year + 1).to_s } }
           end
 
           let(:request) do