New border controls require ‘certificate of entitlement’ to attach to second nationality passport that costs £589

Dual British nationals have been warned they may be denied boarding a flight, ferry or train to the UK after 25 February unless they carry a valid British passport.

The warning by the Home Office comes amid scores of complaints from British people living or travelling abroad who have suddenly found themselves at risk of not being allowed into the UK.

If you are affected by the change and want to share your story, email lisa.ocarroll@theguardian.com