TENS of thousands of drivers could have their speeding convictions quashed after a landmark legal challenge by two police officers.

North Yorkshire Police officers David Burlingham and Andrew McFarlane plunged motoring law into chaos last night after successfully challenging their speeding offences by proving the road signs they drove past were illegal.

The move could open the floodgates for thousands of motorists getting their convictions quashed with police forces forced to pay back millions of pounds in fines.

Lawyers believe that endorsement points and driving bans may also be wiped out.

One motoring group, the Association of British Drivers (ABD), said last night there were some local authorities in Britain which were aware their roads signs were illegal.

Tony Vickers, a spokesman for the ABD, said: "There's a potential tidal wave of drivers who would be entitled to go to court and have their speeding convictions overturned on the means that the speeds were recorded using the wrong legalisation or wrong Road Traffic Act."

Traffic officer Burlingham, 47, was allegedly caught speeding while off duty on the A171 near Guisborough, east Cleveland, in July last year.

It was claimed he was travelling at 58mph in a 50mph zone when he was recorded by a mobile speed camera unit.

A month earlier, PC McFarlane, 35, an officer for 14 years, was allegedly caught on the same stretch of road doing 60mph while driving an unmarked police car.

Both men received notification of the alleged offences from Cleveland Police and each faced a fixed penalty fine of £60 and three points on their driving licences.

But the pair consulted colleague Richard Bentley, an expert in traffic signs, who told them some of the signs installed to warn drivers of the speeding crackdown in the Cleveland Police force area were illegal.

The separate speeding fines against the officers were dropped yesterday when the Crown Prosecution Service offered no evidence against them at Guisborough Magistrates' Court.

The case has plunged Cleveland Police's controversial speed camera scheme into chaos because the signs marking the start of speed enforcement zones are illegal.

Police in Cleveland were among eight areas to pilot the scheme, in April 2000. Thousands of drivers are fined for speeding on target roads across Teesside every month.

The officers were jubilant yesterday when they walked from court after being awarded costs. They declined to comment after the case, but a colleague said: "They are both highly relieved. From the beginning they have maintained the signs were illegal.

"This could lead to thousands of cases where drivers believe they have been wrongly convicted and it could open the floodgates to legal challenges."

Solicitor Gerard Tompkinson, who represented the officers on behalf of the Police Federation, said after the hearing: "Section 85, sub-section four of the Road Traffic Regulation Act says that if the signage regime is not legal and prescribed, nobody can be convicted of exceeding the speed limit."

After the hearing, John Burke, Acting Assistant Chief Constable of Cleveland Police, said the case had been dropped with the utmost reluctance.

"The signs have been deemed unlawful on a pure administrative technicality, and to say that Cleveland Police is disappointed with this is an understatement."

The case follows that of Cleveland Police Detective Superintendent Adrian Roberts, who caused a furore in January 2001 when a speeding ticket was withdrawn after he was unable to say who was driving the car when it was caught on camera.

Motoring groups said there was one law for the police and one for the public.

Richard Freeman, a spokesman for the AA Motoring Trust, said: "If the police were aware that these signs were not enforceable, why didn't they do something about it and tell the council?"

A spokesman for Redcar and Cleveland Borough Council, which is responsible for the signs, said: "We are expecting to start work on replacing all the 16 signs which are considered to be non-prescribed in the very near future.