Half of all lorry drivers stopped for roadside checks in north Wales are driving for illegally long hours, a senior police officer has revealed.
Some truckers are using CB radios to avoid police checks
Chief Superintendent Geraint Anwyl said a "black economy" of freelance truckers were routinely breaching drivers' hours regulations on the route to Holyhead.
Mr Anwyl warned that the problem was likely to be widespread in the UK.
His concerns were exposed during an investigation by BBC Radio 4's File on 4 programme.
Mr Anwyl, head of the National Roads Policing Intelligence Forum, said his force had an ongoing operation to catch truckers who were breaking the law.
He told the BBC: "Half of the vehicles we're stopping on the Late Arrival operation are driving in excess of their hours or they've got tachograph offences or even using forged tachographs."
And Mr Anwyl said it seemed some "disreputable" companies were compelling their drivers to work longer hours while being paid less money.
"A number of months ago a South African (driver) was falling asleep at the wheel and was literally pleading with us to stop him.
"He told us that if he chose to stop himself then he would not be employed again by that company."
Pc Nick Power, a traffic officer with North Wales Police, said some truckers used their CB radios to try to avoid police checks on the A55 to Holyhead.
"Drivers in one particular firm will get together to decide who has the best charts out of three or four wagons," he said.
"They'll send a sacrificial lamb first so you'll see a lorry, you'll pull that one but the ones that are behind will have the real problems with the charts and will thunder through safely to the boats."
Many of the drivers caught in north Wales were from former eastern Bloc countries, the programme found.
Mr Anwyl said many of them had "high standards of driving" but added that they were a labour force about which his officers knew "nothing".
"We have no idea as to the validity of their licences and they seem to be driving regardless of the law regarding rest hours," he said.
But he said that most drivers working for reputable firms, whether from the UK or abroad, abided by the rules.
File on 4 is on BBC Radio 4 at 2000 BST on Tuesday 18 October 2005.