diff --git a/app/controllers/merge_requests_controller.rb b/app/controllers/merge_requests_controller.rb
index dd184457c..6a0662b3d 100644
--- a/app/controllers/merge_requests_controller.rb
+++ b/app/controllers/merge_requests_controller.rb
@@ -1,6 +1,7 @@
 class MergeRequestsController < ApplicationController
-  before_action :authenticate_user!
   before_action :find_resource, only: %i[update organisations update_organisations remove_merging_organisation]
+  before_action :authenticate_user!
+  before_action :authenticate_scope!, except: [:create]
 
   def create
     @merge_request = MergeRequest.create!(merge_request_params)
@@ -72,4 +73,10 @@ private
   def previous_template
     :organisations
   end
+
+  def authenticate_scope!
+    if current_user.organisation != @merge_request.requesting_organisation && !current_user.support?
+      render_not_found
+    end
+  end
 end
diff --git a/spec/requests/merge_requests_controller_spec.rb b/spec/requests/merge_requests_controller_spec.rb
index cafb2df68..cc7aa2893 100644
--- a/spec/requests/merge_requests_controller_spec.rb
+++ b/spec/requests/merge_requests_controller_spec.rb
@@ -8,6 +8,7 @@ RSpec.describe MergeRequestsController, type: :request do
   let(:user) { FactoryBot.create(:user, :data_coordinator) }
   let(:support_user) { FactoryBot.create(:user, :support, organisation:) }
   let(:merge_request) { MergeRequest.create!(requesting_organisation: organisation) }
+  let(:other_merge_request) { MergeRequest.create!(requesting_organisation: other_organisation) }
 
   context "when user is signed in with a data coordinator user" do
     before do
@@ -17,25 +18,49 @@ RSpec.describe MergeRequestsController, type: :request do
     describe "#organisations" do
       let(:params) { { merge_request: { requesting_organisation_id: "9" } } }
 
-      before do
-        organisation.update!(name: "Test Org")
-        post "/merge-request", headers:, params:
+      context "when creating a new merge request" do
+        before do
+          organisation.update!(name: "Test Org")
+          post "/merge-request", headers:, params:
+        end
+
+        it "creates merge request with requesting organisation" do
+          follow_redirect!
+          expect(page).to have_content("Which organisations are merging?")
+          expect(page).to have_content("Test Org")
+          expect(page).not_to have_link("Remove")
+        end
+
+        context "when passing a different requesting organisation id" do
+          let(:params) { { merge_request: { requesting_organisation_id: other_organisation.id } } }
+
+          it "creates merge request with current user organisation" do
+            follow_redirect!
+            expect(MergeRequest.count).to eq(1)
+            expect(MergeRequest.first.requesting_organisation_id).to eq(organisation.id)
+          end
+        end
       end
 
-      it "creates merge request with requesting organisation" do
-        follow_redirect!
-        expect(page).to have_content("Which organisations are merging?")
-        expect(page).to have_content("Test Org")
-        expect(page).not_to have_link("Remove")
+      context "when viewing existing merge request" do
+        before do
+          organisation.update!(name: "Test Org")
+          get "/merge-request/#{merge_request.id}/organisations", headers:, params:
+        end
+
+        it "shows merge request with requesting organisation" do
+          expect(page).to have_content("Which organisations are merging?")
+          expect(page).to have_content("Test Org")
+        end
       end
 
-      context "when passing a different requesting organisation id" do
-        let(:params) { { merge_request: { requesting_organisation_id: other_organisation.id } } }
+      context "when viewing existing merge request of a different (unauthorised) organisation" do
+        before do
+          get "/merge-request/#{other_merge_request.id}/organisations", headers:, params:
+        end
 
-        it "creates merge request with current user organisation" do
-          follow_redirect!
-          expect(MergeRequest.count).to eq(1)
-          expect(MergeRequest.first.requesting_organisation_id).to eq(organisation.id)
+        it "shows merge request with requesting organisation" do
+          expect(response).to have_http_status(:not_found)
         end
       end
     end