diff --git a/app/controllers/external_controller.rb b/app/controllers/external_controller.rb
index ec57eb1006..702fd8633a 100644
--- a/app/controllers/external_controller.rb
+++ b/app/controllers/external_controller.rb
@@ -32,6 +32,13 @@ def create_user
 
     user = User.find_by(external_id: credentials['uid'], provider:)
 
+    # Fallback mechanism to search by email
+    if user.blank? && ENV.fetch('USE_EMAIL_AS_EXTERNAL_ID_FALLBACK', 'false') == 'true'
+      user = User.find_by(email: credentials['info']['email'], provider:)
+      # Update the user's external id to the latest value to avoid using the fallback
+      user.update(external_id: credentials['uid']) if user.present? && credentials['uid'].present?
+    end
+
     new_user = user.blank?
 
     registration_method = SettingGetter.new(setting_name: 'RegistrationMethod', provider: current_provider).call
diff --git a/sample.env b/sample.env
index ecb667e219..d90f6dea9b 100644
--- a/sample.env
+++ b/sample.env
@@ -47,6 +47,10 @@ REDIS_URL=
 #OPENID_CONNECT_REDIRECT=
 #OPENID_CONNECT_UID_FIELD=sub
 
+# Uncomment the following flag if you want to use EMAIL as a Unique ID backup, useful for setups with existing users who want to switch to an IDP setup.
+# More information: https://github.com/bigbluebutton/greenlight/issues/5872
+#USE_EMAIL_AS_EXTERNAL_ID_FALLBACK=true
+
 # To enable hCaptcha on the user sign up and sign in, define these 2 keys
 # More information: https://docs.bigbluebutton.org/greenlight/v3/install/#hcaptcha-setup
 #HCAPTCHA_SITE_KEY=