Police speed guns do make mistakes, the Government has admitted for the first time.

The errors mean that thousands of motorists have potentially lost their licences and even their livelihoods on false grounds.

Almost a million drivers are on the verge of bans after being repeatedly snapped by 3,500 mobile devices across England and Wales.

The Home Office and the police have always insisted that the speed guns - which are held by hand or mounted on a tripod - do not lie.

But that is contradicted by a letter from Geoffrey Biddulph, the senior Home Office civil servant in charge of policing Britain's roads.

In it, he states clearly: "We do accept in certain atypical circumstances a device may be capable of producing inaccurate readings."

The letter was sent to Barry Culshaw, a Hampshire solicitor who has defended scores of motorists unfairly trapped by the cameras.

He has sent the Home Office a damning dossier showing how the cameras and their operators make mistakes. He says that the most unreliable guns, which use laser technology, should now be ditched by the Government.

A Daily Mail investigation last year revealed the full extent of the flaws in the devices.

Our tests showed that the most popular of the guns - the LTI 20.20 Ultralyte 100 - gave erratic results even when operated strictly according to instructions.

It recorded a wall travelling at 44 mph, an empty road doing 33 mph and a parked car managing 22 mph. A bicycle - being ridden at just 5mph - was said to have been doing 66mph.

Most of the mistakes stemmed from the gun's wide beam picking up readings from overtaking cars and parked vehicles. Reflections from the road surface, hoardings, fences and even traffic signs also produced errors.

If the gun was not held firmly on the target - itself a difficult task - 'slippage' led to faulty readings. A movement of as little as the width of a human hair was enough to create mistakes.

Our investigation was monitored by one of the country's leading laser experts, Dr Michael Clark.

Only this week, in what is now being viewed as a landmark case, a motorist accused of speeding at 109mph along the M6 toll road in Staffordshire was cleared by a judge because of an error by a laser gun or its police handlers.

Car salesman Stewart Walker, 37, defended himself at Stoke magistrates court because he could not afford the £1,400 in fees charged by a solicitor.

He told the judge that he was driving at exactly 70mph when he was snapped by an LTI Ultralyte last June.

'This is a huge admission from the Home Office'

As the officer squeezed the trigger Mr Walker was overtaken by a BMW sports car which then pulled in front of his own saloon.

Mr Walker asked the officer, in court to give evidence, if the reflection of the faster car could have been picked up by the camera's laser beam by mistake to give the extraordinarily high reading. The officer said yes.

Mr Walker, from Norfolk, handed the judge a copy of the Mail investigation, published last October. The judge read it before declaring that Mr Walker should have the case against him dropped. His costs will now be paid by the police.

After the hearing, Mr Walker said: "I told the judge that I thought the gun had taken a reading from another car.

"I have been worried for months about losing my licence even though I had done nothing wrong or dangerous at all."

Motoring organisations have consistently said that the cameras are not fault-free.

Some observers have called for speed guns to be scrapped altogether because of the flaws.

Paul Smith, head of Safe-Speed, a group which campaigns against the spread of mobile cameras, said: "This is a huge admission from the Home Office.

"Now the Government has finally confessed that a problem exists, they must withdraw the devices and make arrangements to compensate those convicted or fined on the basis of unreliable evidence.

"Five million motorists have been convicted or paid a fixed penalty in the past five years. Now we know that many have not broken any law at all."

Mark McArthur-Christie, policy director of the British Driving Association, said: "The public are beginning to mistrust the police because of the unfairness of the cameras which trap the innocent."

More than two million motorists are expected to receive speed camera tickets this year. If each is fined £60, the total profits from the cameras in 2005-2006 will be £118million. The money goes to police forces, road safety groups and courts that hear speeding cases.

The Home Office declined to comment.