Registrars in Croydon report an average of three suspected sham marriages a month to immigration officials, it has been revealed.

However, it is thought as many as two suspect marriages were being reported every day in London boroughs before immigration laws were tightened eight years ago.

Last week, an address in Thornton Heath was one of three properties in the UK targeted in police raids, tackling sham marriages which exploit immigration laws.

But it is now much harder to use a bogus marriage as a short cut for a UK passport, according to Croydon deputy superintendant registrar, Richard Evans.

"After 20-odd years in this job you get to know the signs of a couple who might not be genuine," said Mr Evans, who is based at Croydon's registry office in Katharine Street. "Laws introduced in 1999 mean there are now a lot more paperwork and requirements to meet.

"If a couple is missing any of these components we are required by law to notify the Home Office, which places an alert on the couple.

"This might not affect them right away but, when one of them applies for a visa a couple of years down the line, their marriage comes under intense scrutiny."

Although no formal statistics on sham marriages are kept in the borough, Mr Evans estimates Croydon registrars report between two and four suspect couples a month.

He added: "Before 1999 it was much more than that. Although I wasn't working in Croydon back then, the average London borough would report one or two suspected sham marriages a day.

"Back then registrars would see the same old faces and names cropping up. For instance, if a couple were married and the groom would then act as the witness for another sham marriage.

"The tell-tale signs we look out for are documents which look forged or couples who have not got the necessary paperwork. Of course there are the more obvious signs. Couples who don't know the basic facts about their spouse-to-be, or those whose body language makes it obvious they are not together.

"And there are also the ones who go too over-the-top and walk in with their hands all over each other.

"Then again, I've dealt with many British couples who don't know the first thing about their partners either."