diff --git a/app/models/user.rb b/app/models/user.rb
index f9e34a652..4e762c9a1 100644
--- a/app/models/user.rb
+++ b/app/models/user.rb
@@ -24,6 +24,7 @@ class User < ApplicationRecord
   after_validation :send_data_protection_confirmation_reminder, if: :is_dpo_changed?
 
   validates :organisation_id, presence: true
+  validate :organisation_not_merged
 
   has_paper_trail ignore: %w[last_sign_in_at
                              current_sign_in_at
@@ -185,6 +186,12 @@ protected
 
 private
 
+  def organisation_not_merged
+    if organisation&.merge_date.present? && organisation.merge_date < Time.zone.now
+      errors.add :organisation_id, I18n.t("validations.organisation.merged")
+    end
+  end
+
   def send_data_protection_confirmation_reminder
     return unless persisted?
     return unless is_dpo?
diff --git a/config/locales/en.yml b/config/locales/en.yml
index e3507b9e5..09abf6729 100644
--- a/config/locales/en.yml
+++ b/config/locales/en.yml
@@ -198,6 +198,7 @@ en:
       managing_agent:
         blank: "You must choose a managing agent"
         already_added: "You have already added this managing agent"
+      merged: "Select an organisation that has not been merged"
     not_answered: "You must answer %{question}"
     invalid_option: "Enter a valid value for %{question}"
     invalid_number: "Enter a number for %{question}"