Police claim more and more car number plates are being stolen from Oxford car parks and used by criminals to avoid London's congestion charge.

Staff at the city crime prevention unit said thieves were preying on vehicles in big car parks, such as park-and-ride sites, Oxford Brookes University and hospitals.

It is believed they use the plates to change the identity of another car, or sell them on a lucrative black market for the same purpose.

Changing the identity of a car means criminals can avoid detection by CCTV and enforcement cameras, such as those in central London which can pinpoint drivers who have failed to pay the daily £8 charge to travel into the city.

To combat the problem, Oxford crime reduction advisers Nick Gilbert and Angie Shuck have launched the safe plate scheme.

The project aims to raise awareness of the crime among motorists, and offers them anti-theft screws to secure their number plates.

Since the start of the campaign, they have seen anecdotal reports of registration plate theft drop from four or five a week, to one or two.

There are no specific statistics for the crime because it is recorded by police as general car theft.

Mrs Shuck said: "Car registration plate theft is common in places where cars are left unattended for a long time, like park-and-rides.

"Stolen plates are used in crimes like bilking - where someone fills up at a petrol station and drives off without paying. Although the car is caught on CCTV, they aren't identified.

"More and more plates are being used to avoid congestion charges in London, so we do anything we can to stop that from happening."

The crime prevention team has spent time at different car parks offering to fit anti-theft screws free of charge.

The simple device makes it difficult for thieves to remove the plates.

Mrs Shuck said: "Two weeks ago, we went to the John Radcliffe Hospital in Headington and did an event early in the morning, where we offered motorists anti-theft screws for their registration plates.

"It was so successful we're looking to do it again at the Churchill and Nuffield Orthopaedic Centre."

Despite Thames Valley Police's proactive measures to combat the problem, campaigners said more traffic police on the roads was the only thing that would deter thieves stealing the identity of other cars.

Edmund King, director of motoring think-tank, the RAC Foundation, said: "Many people don't even know number plate theft has happened and just think they've lost their plates.

"There's not much the individual driver can do about it.

"Even anti-theft devices don't solve the problem, because there are 30,000 businesses that can make plates, and it's difficult to regulate them."