An Oxford man is selling a spray he claims makes number plates invisible to speed cameras.

Dave Pollard, 69, of Botley, says the spray, called Photoblock, over-exposes photographs taken by cameras, enabling motorists to avoid fines or penalty points on their licence. But Thames Valley Police -- which rakes in more money in speeding fines than any other force in the country -- has questioned the effectiveness of the spray, and warned drivers they could be charged with perverting the cause of justice.

Mr Pollard said: "You spray it in on your number plate, leave to set, and it goes hard like nail polish. The spray contains crystals which reflect the flash and over-expose the photograph.

"Just one spray is enough to last the car's lifetime and there is enough in one can for six cars."

Photoblock was developed in Oxford and is manufactured in the US, where half a million cans have been sold. Mr Pollard claims he is the only sales outlet for the £25.95 spray in the UK, although it is available over the Internet.

He said: "I don't mind about cameras being put near schools, in villages or in accident blackspots, but many are on the roads simply to earn the Government money, which I disagree with. Why should people have to pay money and risk losing their licence just because they are driving a few miles over the speed limit on an unfamiliar road?"

Thames Valley Police has tested various sprays and lacquers and says they do not work. Spokeswoman Victoria Bartlett said: "If the spray does not work, the seller could face the charge of obtaining money by deception. If the spray is effective, the buyer would be attempting to pervert the course of justice."

Drivers caught speeding are fined £60 and have three points added to their licence, although motorists sometimes have the option of attending a speed awareness course.

Mark McArthur-Christie, of the British Association of Drivers, said "We can understand the frustrations of people who try to evade speed cameras but we certainly do not condone this practice."

Thames Valley Police raked in £4.6m from speed camera fines -- £2m going to Government coffers -- during 2001-2. But accidents within 500m of sites increased by 14 per cent, compared to a 35 per cent reduction nationally.

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