Simon Graham's marital bliss would face a glitch if he gets hitched because an identity mix-up means he is already registered as married and living with his wife and three children in the north of England.

During the past three years, the 31-year-old welder from Starwort Path in Blackbird Leys, Oxford, has received mail meant for another Simon Graham who shares the same name, date of birth and national insurance number.

Letters reveal his alter-ego is married to a woman called Tina, has three children and lives in Cumbria, where he works in a paper mill.

Oxford's Simon Graham is unsure whether it is an innocent mix-up or an identity theft, but fears marrying his partner for the past six years, Nicola Harris, could result in being arrested for bigamy.

The father-of-two said: "What are the chances of someone being born on the same day with the same name walking round with the same national insurance number?

"It's very spooky. We went through the phone book trying to find a Tina and Simon Graham in Cumbria, but couldn't track them down.

"What's worrying me is that we are thinking about a wedding. I could be arrested for bigamy if I marry Nicola, because I'm supposed to be already married."

Letters arrived on Mr Graham's doorstep in 2003, agreeing to the payment of Family Tax Credits for his mystery wife and three children.

He took the letters to the Job Centre, Inland Revenue and Family Tax Credits but letters continued to arrive.

Now he will be issued with a brand new national insurance number, but fears contributions made over his working life and future applications for Family Tax Credits could be jeo- pardised.

Customs and Excise officers are investigating whether he has been a victim of identity theft.

The Inland Revenue initially told Mr Graham it was a cross-identity and the Family Tax Credits office said it would never happen again. But the mistake may have been made when the National Insurance numbers were issued 15 years ago.

A spokesman for Customs and Excise said it could not discuss individual cases, but added: "We take the integrity of the National Insurance number very seriously, and will ensure that NI contributions paid by a customer are recorded on the correct account.

"National insurance payments are protected and there is no question of any customer losing future entitlements."

To register for marriage, Mr Graham will have to supply identification and confirmation of his nationality.

Alicja Gilroy, superintendent register for Oxfordshire, said: "As long as he comes to me and can honestly declare he has not been through formal marriage, he should be fine."