diff --git a/app/contracts/project_custom_field_project_mappings/base_contract.rb b/app/contracts/project_custom_field_project_mappings/base_contract.rb
index 106abf73b07f..0ed19cf6d5a1 100644
--- a/app/contracts/project_custom_field_project_mappings/base_contract.rb
+++ b/app/contracts/project_custom_field_project_mappings/base_contract.rb
@@ -46,7 +46,7 @@ def not_required
       # enabling a custom field which is required happens in an after_save hook within the custom field model itself
       return if model.project_custom_field.nil? || !model.project_custom_field.required?
 
-      errors.add :custom_field_id, :invalid
+      errors.add :custom_field_id, :cannot_delete_mapping
     end
 
     def visbile_to_user
diff --git a/app/controllers/admin/settings/project_custom_fields_controller.rb b/app/controllers/admin/settings/project_custom_fields_controller.rb
index a23139a70ddc..188395ad404a 100644
--- a/app/controllers/admin/settings/project_custom_fields_controller.rb
+++ b/app/controllers/admin/settings/project_custom_fields_controller.rb
@@ -30,6 +30,8 @@ module Admin::Settings
   class ProjectCustomFieldsController < ::Admin::SettingsController
     include CustomFields::SharedActions
     include OpTurbo::ComponentStream
+    include ApplicationComponentStreams
+    include FlashMessagesOutputSafetyHelper
     include Admin::Settings::ProjectCustomFields::ComponentStreams
 
     menu_item :project_custom_fields_settings
@@ -41,6 +43,7 @@ class ProjectCustomFieldsController < ::Admin::SettingsController
     before_action :prepare_custom_option_position, only: %i(update create)
     before_action :find_custom_option, only: :delete_option
     before_action :project_custom_field_mappings_query, only: %i[project_mappings unlink]
+    before_action :find_unlink_project_custom_field_mapping, only: :unlink
     # rubocop:enable Rails/LexicallyScopedActionFilter
 
     def show_local_breadcrumb
@@ -68,13 +71,18 @@ def edit; end
     def project_mappings; end
 
     def unlink
-      project = Project.find(permitted_params.project_custom_field_project_mapping[:project_id])
-      project_custom_field_mapping = @custom_field.project_custom_field_project_mappings.find_by(project:)
       delete_service = ProjectCustomFieldProjectMappings::DeleteService
-                         .new(user: current_user, model: project_custom_field_mapping)
+                         .new(user: current_user, model: @project_custom_field_mapping)
                          .call
 
-      delete_service.on_success { render_unlink_response(project:) }
+      delete_service.on_success { render_unlink_response(project: @project) }
+
+      delete_service.on_failure do
+        update_flash_message_via_turbo_stream(
+          message: join_flash_messages(delete_service.errors.full_messages),
+          full: true, dismiss_scheme: :hide, scheme: :danger
+        )
+      end
 
       respond_to_with_turbo_streams(status: delete_service.success? ? :ok : :unprocessable_entity)
     end
@@ -150,6 +158,23 @@ def set_sections
                                          .all
     end
 
+    def find_unlink_project_custom_field_mapping
+      @project = Project.find(permitted_params.project_custom_field_project_mapping[:project_id])
+      @project_custom_field_mapping = @custom_field.project_custom_field_project_mappings.find_by!(project: @project)
+    rescue ActiveRecord::RecordNotFound
+      update_flash_message_via_turbo_stream(
+        message: t(:notice_file_not_found), full: true, dismiss_scheme: :hide, scheme: :danger
+      )
+      replace_via_turbo_stream(
+        component: Settings::ProjectCustomFields::ProjectCustomFieldMapping::TableComponent.new(
+          query: project_custom_field_mappings_query,
+          params: { custom_field: @custom_field }
+        )
+      )
+
+      respond_with_turbo_streams
+    end
+
     def find_custom_field
       @custom_field = ProjectCustomField.find(params[:id])
     rescue ActiveRecord::RecordNotFound
diff --git a/app/helpers/flash_messages_helper.rb b/app/helpers/flash_messages_helper.rb
index 0fc868657ed8..36321e3ae4f7 100644
--- a/app/helpers/flash_messages_helper.rb
+++ b/app/helpers/flash_messages_helper.rb
@@ -31,8 +31,7 @@ module FlashMessagesHelper
   extend ActiveSupport::Concern
 
   included do
-    # For .safe_join in join_flash_messages
-    include ActionView::Helpers::OutputSafetyHelper
+    include FlashMessagesOutputSafetyHelper
   end
 
   def render_primer_banner_message?
@@ -45,6 +44,11 @@ def render_primer_banner_message
     render(BannerMessageComponent.new(**flash[:primer_banner].to_hash))
   end
 
+  # Primer's flash message component wrapped in a component which is empty initially but can be updated via turbo stream
+  def render_streameable_primer_banner_message
+    render(FlashMessageComponent.new)
+  end
+
   # Renders flash messages
   def render_flash_messages
     return if render_primer_banner_message?
@@ -63,14 +67,6 @@ def render_flash_messages
     safe_join messages, "\n"
   end
 
-  def join_flash_messages(messages)
-    if messages.respond_to?(:join)
-      safe_join(messages, "<br />".html_safe)
-    else
-      messages
-    end
-  end
-
   def render_flash_message(type, message, html_options = {}) # rubocop:disable Metrics/AbcSize
     if type.to_s == "notice"
       type = "success"
diff --git a/app/contracts/project_custom_field_project_mappings/delete_contract.rb b/app/helpers/flash_messages_output_safety_helper.rb
similarity index 78%
rename from app/contracts/project_custom_field_project_mappings/delete_contract.rb
rename to app/helpers/flash_messages_output_safety_helper.rb
index 7392d8d7bdf0..563079419797 100644
--- a/app/contracts/project_custom_field_project_mappings/delete_contract.rb
+++ b/app/helpers/flash_messages_output_safety_helper.rb
@@ -25,9 +25,21 @@
 #
 # See COPYRIGHT and LICENSE files for more details.
 #++
+#
+
+module FlashMessagesOutputSafetyHelper
+  extend ActiveSupport::Concern
+
+  included do
+    # For .safe_join in join_flash_messages
+    include ActionView::Helpers::OutputSafetyHelper
+  end
 
-module ProjectCustomFieldProjectMappings
-  class DeleteContract < ::DeleteContract
-    delete_permission :select_project_custom_fields
+  def join_flash_messages(messages)
+    if messages.respond_to?(:join)
+      safe_join(messages, "<br />".html_safe)
+    else
+      messages
+    end
   end
 end
diff --git a/app/services/project_custom_field_project_mappings/delete_service.rb b/app/services/project_custom_field_project_mappings/delete_service.rb
index 3e0b4501a185..5c3a03feef8c 100644
--- a/app/services/project_custom_field_project_mappings/delete_service.rb
+++ b/app/services/project_custom_field_project_mappings/delete_service.rb
@@ -28,5 +28,10 @@
 
 module ProjectCustomFieldProjectMappings
   class DeleteService < ::BaseServices::Delete
+    # Mappings have custom deletion rules that are similar to the update rules all derived from the base contract
+    # Reuse the update contract to ensure that the deletion rules are consistent with the update rules
+    def default_contract_class
+      ProjectCustomFieldProjectMappings::UpdateContract
+    end
   end
 end
diff --git a/app/views/layouts/base.html.erb b/app/views/layouts/base.html.erb
index a311e7f001d4..4f844ca25f2c 100644
--- a/app/views/layouts/base.html.erb
+++ b/app/views/layouts/base.html.erb
@@ -118,6 +118,7 @@ See COPYRIGHT and LICENSE files for more details.
     <div class="content-overlay"></div>
     <main id="content-wrapper" class="<%= initial_classes %>">
       <%= render_primer_banner_message %>
+      <%= render_streameable_primer_banner_message %>
       <% if show_decoration %>
         <div id="breadcrumb" class="<%= initial_classes %><%= show_breadcrumb ? ' -show' : '' %>">
           <%= you_are_here_info %>
diff --git a/config/locales/en.yml b/config/locales/en.yml
index 8c3d8b8f422b..7a4208389fb1 100644
--- a/config/locales/en.yml
+++ b/config/locales/en.yml
@@ -831,6 +831,7 @@ Project attributes and sections are defined in the <a href=%{admin_settings_url}
         before_or_equal_to: "must be before or equal to %{date}."
         blank: "can't be blank."
         blank_nested: "needs to have the property '%{property}' set."
+        cannot_delete_mapping: "is required. Cannot be deleted."
         cant_link_a_work_package_with_a_descendant: "A work package cannot be linked to one of its subtasks."
         circular_dependency: "This relation would create a circular dependency."
         confirmation: "doesn't match %{attribute}."
diff --git a/modules/storages/app/controllers/storages/admin/storages_controller.rb b/modules/storages/app/controllers/storages/admin/storages_controller.rb
index 7c7983b3d81b..bacb294a174d 100644
--- a/modules/storages/app/controllers/storages/admin/storages_controller.rb
+++ b/modules/storages/app/controllers/storages/admin/storages_controller.rb
@@ -32,7 +32,7 @@
 class Storages::Admin::StoragesController < ApplicationController
   using Storages::Peripherals::ServiceResultRefinements
 
-  include FlashMessagesHelper
+  include FlashMessagesOutputSafetyHelper
   include OpTurbo::ComponentStream
 
   # See https://guides.rubyonrails.org/layouts_and_rendering.html for reference on layout
diff --git a/spec/contracts/project_custom_field_project_mappings/base_contract_spec.rb b/spec/contracts/project_custom_field_project_mappings/base_contract_spec.rb
new file mode 100644
index 000000000000..c61861522cbd
--- /dev/null
+++ b/spec/contracts/project_custom_field_project_mappings/base_contract_spec.rb
@@ -0,0 +1,73 @@
+# frozen_string_literal: true
+
+#-- copyright
+# OpenProject is an open source project management software.
+# Copyright (C) 2012-2024 the OpenProject GmbH
+#
+# This program is free software; you can redistribute it and/or
+# modify it under the terms of the GNU General Public License version 3.
+#
+# OpenProject is a fork of ChiliProject, which is a fork of Redmine. The copyright follows:
+# Copyright (C) 2006-2013 Jean-Philippe Lang
+# Copyright (C) 2010-2013 the ChiliProject Team
+#
+# This program is free software; you can redistribute it and/or
+# modify it under the terms of the GNU General Public License
+# as published by the Free Software Foundation; either version 2
+# of the License, or (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program; if not, write to the Free Software
+# Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
+#
+# See COPYRIGHT and LICENSE files for more details.
+#++
+
+require "spec_helper"
+require "contracts/shared/model_contract_shared_context"
+
+RSpec.describe ProjectCustomFieldProjectMappings::BaseContract do
+  include_context "ModelContract shared context"
+
+  let(:contract) { described_class.new(project_custom_field_mapping, user) }
+  let(:user) { build_stubbed(:admin) }
+  let(:project) { build_stubbed(:project) }
+  let(:project_custom_field) { build_stubbed(:project_custom_field) }
+  let(:project_custom_field_mapping) { build_stubbed(:project_custom_field_project_mapping, project:, project_custom_field:) }
+
+  before { User.current = user }
+
+  context "when the custom field is required" do
+    let(:project_custom_field) { build_stubbed(:project_custom_field, is_required: true) }
+
+    it_behaves_like "contract is invalid"
+  end
+
+  context "with non-visible custom field and admin user" do
+    let(:project_custom_field) { build_stubbed(:project_custom_field, visible: false) }
+
+    before do
+      allow(ProjectCustomField).to receive(:all).and_return([project_custom_field])
+    end
+
+    it_behaves_like "contract is valid"
+  end
+
+  context "with non-visible custom field and non-admin user" do
+    let(:user) { build_stubbed(:user) }
+    let(:project_custom_field) { build_stubbed(:project_custom_field, visible: false) }
+
+    before do
+      allow(ProjectCustomField).to receive(:where).with(visible: true).and_return([project_custom_field])
+    end
+
+    it_behaves_like "contract is invalid"
+  end
+
+  include_examples "contract reuses the model errors"
+end
diff --git a/spec/services/project_custom_field_project_mappings/delete_service_spec.rb b/spec/services/project_custom_field_project_mappings/delete_service_spec.rb
index e81be32e838f..4520872b8186 100644
--- a/spec/services/project_custom_field_project_mappings/delete_service_spec.rb
+++ b/spec/services/project_custom_field_project_mappings/delete_service_spec.rb
@@ -32,5 +32,8 @@
 RSpec.describe ProjectCustomFieldProjectMappings::DeleteService do
   it_behaves_like "BaseServices delete service" do
     let(:factory) { :project_custom_field_project_mapping }
+    let(:contract_class) do
+      "#{namespace}::UpdateContract".constantize
+    end
   end
 end