The Metropolitan Police Fraud Squad is warning of a new twist to an old scam as crooks turn to the internet to help them con people out of thousands of pounds.

Similar to the so-called 'Nigerian' or '419' email scams, it starts off in the old-fashioned way via classifieds in UK newspapers.

The crooks look for victims who are advertising items for sale, most often cars. Initially they phone, posing as dealers who have a client on the lookout for just the item the target is advertising.

After initial phone contact, the crooks move the transaction to the internet and communicate via email, claiming that they need more details of the item, including photographs, to give their 'client'.

This allows the criminals to cut the cost of the scam and better disguise their identities.

Once the victim's trust has been gained and a 'sale' is imminent, the crooks then tell the seller that there is a problem.

The client they are acting for was going to buy a similar item, for example the same make and model of car, from another vendor which was more expensive, but has been let down. This means that the cheque has been made out for more than the selling price.

The deal they offer is to send the cheque to the seller, asking for the excess - normally around £2,000 to £3,000 - to be wired back to them.

But the cheques are stolen, and bounce several days after they are cashed, often well after the victim has wired the excess to the requested destination.

According to Detective Sergeant Colin Holder of the Met's Fraud Squad, which was alerted to the scam about two weeks ago, an increasing number of people are being duped.

"This scam has all the hallmarks of the West African frauds because of its style; the crooks saying that they are brokers sending the car to a client in Africa and the use of Western Union," he told vnunet.com.

"It shows how the internet is a powerful tool for criminals as we are seeing a real increase in the number of people falling for this scam since we were alerted to its existence a couple of weeks ago."