This is the heart-stopping moment every motorist dreads. As you drive along the road, a police officer points a laser speed gun towards you. Glancing at the dashboard, you breathe a sigh of relief: the speedometer reveals that your car is travelling below the 30mph limit. But a month later, a letter drops through your door. You face a fine for speeding and penalty points on your licence.

It is claimed that you were driving at 41mph – not 28mph. Can that high speed really be true? Staggeringly, the answer may be no.

Motorists accused of driving too fast on Britain's roads insist the real culprit is a laser speed gun officially approved by the Home Office and used by almost every police authority in the country. For the Mail has discovered that the LTI 20.20. gun is seriously flawed.

In our tests, it wrongly recorded a wall as travelling at 44mph, an empty road scored 33mph, a parked car was clocked as doing 22mph and a bicycle (in reality being ridden at 5mph) rocketed along at an impossible 66mph.

Imported from America, the LTI 20.20. is used in nearly 3500 mobile speed units hidden in police vans or cars and mounted on motorbikes.

Speed traps – nearly half of which now use laser gun technology – reap more than £100million each year in fines. This is shared between the police, the Highways Agency, the courts, the Home Office and local authorities. Ironically, some of the huge sum is used to pay for even more police speed reinforcement teams relying on exactly the same laser speed gun at the centre of the Mail's investigation.

We subjected the speed gun to rigorous tests. Alarmingly, we discovered it was prone to wildly wide-of-the-mark readings, even when set up according to the police's own guidelines and the manufacturer's handbook.

In other tests, we found the equipment was measuring the speed of overtaking cars instead of the one being targeted.

Today, The Mail can expose the scandal of a speed-enforcement industry in which the collection of fines is considered paramount – whatever the consequences for innocent drivers caught in police traps by faulty readings.

In the past nine years, an extraordinary one-in-five drivers has been fined for speeding, despite many protesting their innocence.

Lawyers we spoke to say motorists are now rebelling by refusing to pay fines and fighting their cases through the courts.

One voicing concern is Barry Culshaw, a Southampton solicitor currently acting for 15 drivers nationwide. “They complain of huge errors,” he says. “Drivers say they were within the speed limit and yet the LTI 20.20 recorded them doing excessive speed.”

Another disquieting discovery is that vital video film – often taken at a speed-trap site for use as secondary evidence – is often mysteriously withheld from motorists by the Crown Prosecution Service.

On at least ten occasions the Crown has suddenly dropped the case against a motorist when ordered by a judge to hand over the telling footage.

Michael Morgan, who runs a British website collating complaints against laser guns, said: “The authorities often wriggle rather than release the video, which would expose the laser gun to scrutiny in a court of law. No doubt they fear the enormous consequences, including a clamour for fine refunds and compensation over the loss of licences or even livelihoods.”

Alarmingly, the Mail can reveal, too, that the main expert witness used by the CPS to convict motorists in such cases – a former police officer named Frank Garratt – also makes his living as boss of the company importing the devices into Britain. Perhaps not surprisingly, Mr Garratt, a millionaire, told the Mail the LTI 20.20 works perfectly well.

One of the gun's toughest critics is Dr Michael Clark, Europe's leading expert on laser technology. He is a former company director of a British firm making laser detection equipment for traffic lights and car parks.

Dr Clark was clocked, apparently speeding, by a laser gun three years ago. He fought his case through the courts, proving he was travelling below the limit. He has acted as an expert witness on behalf of many motorists since.

“I was drawn into this controversy because I know about laser science. I do not rely on my court appearances or the speed enforcement industry to make a living.” He told the Mail when we asked him to help – without payment – in our experiments.

Dr Clark says that the gun is defective because its wide beam can easily pick up the wrong vehicle. Furthermore, if the device is not held firmly on the target – and this is a difficult task – it can produce an erroneous speed result by ‘slippage'.

Reflections from road signs and from other cars – even one stationary on the kerbside – can also make the laser gun misinterpret the truth.

Several types of laser gun are used in Britain but all are built on precisely the same principle. Among the most popular is the LTI 20.20. Ultralyte, which is the model we tested.

It is made by Laser Technology Inc and imported here by Tele-Traffic, whose managing director is Mr Garratt. Ninety-seven per cent of police forces use speed enforcement systems supplied by his company. This is how it works: The police officer – or, increasingly, a civilian operator – spots a vehicle suspected of speeding. Then, using the laser gun (normally mounted on a tripod and swung around manually in order to track the vehicle) he triggers a button to send out an infra-red beam.

To do this accurately, he looks through the sights on top of the gun and sees a red dot. He aims this dot at the target and squeezes the trigger. This releases the invisible beam which, in a fraction of a second, is sent back to the gun when it reaches the speeding vehicle. It does this again and again.

By measuring the amount of time it takes for the beam to bounce back, the device can determine the distance to the target and, in turn, the vehicle's speed towards or away from the gun.

Significantly, the Association of Chief Police Officers' (ACPO) own code of practice says that users of the LTI 20.20. must check the sights and readings of the gun every time they set up a speed trap.

Critically, the beam must be held firmly on the vehicle, preferably the number plate. But if ‘slippage' occurs and the beam moves up or along the car, the gun can be tricked into a false reading.

For instance, tests in the US have shown that if the beam slips from the windscreen of the car down to the grille of the bonnet, this can add on 8mph. Astonishingly, if the beam slips along the entire length of the car – as is possible when a vehicle comes around a corner into the speed gun's sights – an erroneous 30mph can be added to the reading.

The BBC programme Inside Out recently revealed a whole series of discrepancies with the LTI 20.20. Worryingly, one leading British engineer told the BBC that errors are provoked if the operator lets the gun in his hands move even ‘the thickness of a human hair.'

The BBC's findings were dismissed by Tele-Traffic and the police. Mr Garratt said it was impossible for the gun to make an error when it tracked a moving car because technology corrected the fault. ACPO added that the BBC experiment was misleading because the US version of the gun was used with less reliable software to reveal errors than the British version.

But the Mail's investigation has proved beyond doubt that the laser gun IS unreliable.

We tested the British version of the gun – an LTI 20.20. Ultralyte certificated by the Tele-Traffic headquarters in Warwick – to allay any criticisms. We also tried the gun on both stationary and moving vehicles: a white transit van, a yellow Ford, a silver Corsa and a bicycle.

We conducted our tests on Wednesday of this week on a quiet industrial estate in a British city.

First, we checked the laser gun and its sights in accordance with the operating handbook and police guidelines.

As we mounted the LTI 20.20. Ultralyte on a tripod, Dr Clark pointed out: “The laser used is a multi-mode laser. If it was projected on to a nearby surface it would reveal three beams, not one. Over a long distance, the beams widen. If the laser is targeted at one vehicle, it can – unwittingly – hit another nearby vehicle by mistake.

“A policeman can't tell from 400 metres away – or an even longer distance in many cases – exactly which car he is marking.”

To demonstrate his point, we parked the Corsa at the side of the road and drove the white van past it at just 3mph. The laser gun was being targeted at the Corsa from 371.7 metres away, which – according to the handbook for the gun – is easily within its capabilities.

But the minute the white van overtook, the laser gun beam picked up the moving van. It was impossible to keep it registered only on the parked Corsa which, as a result, was wrongly recorded as moving at 3mph. We repeated the experiment at 26mph and the gun showed the stationary Corsa moving at 26mph.

One big problem is reflection from other cars. We pointed the gun at the Ford which was parked on the roadside and slowly drove the white van past. Bizarrely, the gun recorded the motionless Ford as moving at 22mph.

“The reason is that the gun's technology presumes that there is always a direct line between the police operator and the speeding vehicle, “ explained Dr Clark.

“But the beam sometimes catches the reflection of a nearby car. It zigzags to this car before carrying on to the target vehicle and returns by the same route.”

And it is this zigzag from point A (the gun) to B (the white van) via C (the reflection on the side panel of the Ford) that distorts the speed reading.

Finally, we targeted the red dot of the LTI 20.20. Ultralyte at a wall on a trading estate. There were no moving cars in the vicinity. We moved the beam along the wall instead of holding it completely steady on a single point, so creating ‘slippage.' The gun was confused into giving a reading of 44mph.

“This shows how a traffic officer can mistakenly pick up a reading from a wall by the side of a motorway or even an empty road if he does not target a vehicle properly,” said Dr Clark.

There is another problem concerning the LTI 20.20. Ultralyte. ACPO says that ideally equipment should not be used over more than 500 metres, although according to Dr Clark, some vehicles are being targeted at double that distance.

However, the shortcomings of this gun were first discovered nearly a decade ago. In 1966, the State of New Jersey in America temporarily barred the LTI 20.20. after a case involving a motorist accused of speeding.

The man's lawyer, Joe Maccarone, said: “During a break, the LTI 20.20. was sitting on a desk. One of our experts turned it on and pointed to the corner of the room. Then, as he pulled the trigger, he moved the gun sharply. It showed the wall was travelling at 4mph.”

Maccarone also summoned a NASA laser scientist who said that just 300 metres, there was only a 60 per cent chance of a human operator hitting a 12ft-wide target with a laser gun. “Yet cars are only 6ft wide. So the chances of hitting something other than the targeted car are very high indeed.”

We put our findings to Mr Garratt. He told the Mail: “The equipment is Home Office Approved, and, when used in accordance with manufacturer's instructions and ACPO's Code Of Practice, presents no problems.”

When we asked him about the ethics of him being called as an expert witness for the prosecution when he also markets the gun, he refused to comment. The Home Office said it was normal to ask a supplier of public service equipment to give evidence in court on behalf of the Government.

Meredydd Hughes, ACPO spokesperson on road policing enforcement technology and Chief Constable of South Yorkshire, had ‘full confidence in the device.'

But the doubters refuse to be silences. Paul Smith, founder of Safe Speed – one of the leading road awareness campaigns – has demanded that the Home Office suspends approval of the laser gun pending an independent investigation.

Ironically, the failings of the gun, which can read too low as well as too high, mean thousands of motorists who should have been caught for speeding have not been punished.

Meanwhile, there are plenty of people waiting to challenge the gun's accuracy. Last December, Alastair Slade, a 37-year-old London accountant, was driving down the A306 in Wiltshire towards Portsmouth. He claims his speed was within the 70mph limit. But he was clocked by the LTI 20.20. at 78mph.

He has requested the video of his ‘crime' from police six times. But to date they have failed to give it to his lawyers.

“I have been accused of speeding, but I am being denied the opportunity of defending myself because I cannot see the video which could clear me. I am going to court to ask for this film evidence next week.”

He is following in the footsteps of Michael Hall, who was clocked in Southampton by the laser gun. “I'm convinced I was doing under 30mph, because I checked my speed,” he insists. “The police said I was travelling at 41mph.”

Mr Hall managed to get video footage of the incident, which he says proved there were errors. He was planning to use this footage to fight the speeding claim, yet without warning, the case against him was suddenly discontinued.

Of course, this won't surprise the enormous number of motorists caught speeding by the now infamous LTI 20.20. laser gun who insist they have done nothing wrong.

They, and their lawyers, say such a travesty will end only when this dubious piece of equipment is banned from roadsides.

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