The BBC's Yigal Chazan reports: "Police fear the law is being abused"

The animal welfare charity says American Bulldogs are being imported and cross-bred in the UK to produce fighting dogs that are potentially extremely aggressive in an effort to side-step current legislation.

The RSPCA's Chief Vetenary Officer Chris Lawrence said these new breeds could pose a serious danger to the public.

"This has all the potential to be equally as great a problem as the Pitbull was. We have at least legislation to control them. What we have here is dogs which are potentially really nasty and really vicious."

There are fears that more breeds will be blacklisted

The police are urging the government to tackle the problem.

They say they have evidence that criminals are using the cross-breeds to side-step the current law on dangerous dogs.

Some of the scores of American Bulldogs brought over to the UK have been bought by reputable rare-breed enthusiasts. But Sgt Ian McFarlan, a dangerous dogs expert with the Metropolitan Police, said there were signs that others are misusing the breed.

"There's been quite a dramatic rise in newspaper advertisements offering for sale American Bulldogs, crossed with Irish Staffordshires - a well-known old euphemism for a Pitbull Terrier," he said.

"If they're bred correctly for temperament then their temperament is certainly going to deteriorate in the same way as it did when pure-bred Pitbulls were crossed with Staffordshires in the 1980s and 1990s."

Skirting around the law

The 1991 Dangerous Dogs Act was introduced after a spate of vicious attacks on young children and is designed to make it almost impossible to own a Pitbull without complying with strict regulations.

But this new fighting dog could fall outside the remit of current legislation.

Reputable breeders are against such cross-breeding for fear it will tar them with the same brush and lead to laws that outlaws breeding of any kind of fighting dog.

And owners are worried their own dogs may fall under the dogs blacklist if stricter rules were introduced.

"If any breed were added to the Dangerous Dogs Act," said Caroline Kisko of the Kennel Club, "then you are outlawing that type of breed whereas in actual fact there are very many nice dogs within the American Bulldog breed."

The National Canine Defence League believes it is the owners not the dogs that should be blacklisted.

"Maybe it's a question of finding out who's importing these dogs and the reasons why they're importing them and who they're being sold on to," said the organisation's Chief Executive Clarissa Baldwin.

"We would like to see breeders of this type of dog take a lot of care and a lot of scrutiny over the people that they're selling them to."

The Home Office is aware of police concerns over the American Bulldog. A spokesman said the situation would be monitored closely and it would consult animal welfare groups and dog associations to assess what action should be taken.