As has been advised, send it back ASAP.

If you have the indignity to also loose your passport, be prepared to part with enough information identify you and send that off too (ideally with your BP stub unless you've lost it) and make copies so that when you get to the Border, you can prove that you actually left the country and willingly wanted to return.

Amazingly, I had no problems and was let through after a couple of minutes, explaining to the poor trainee customs agent and supervisor. I think my Brummy/Essex accent baffled them eventually