A growing number of lorry drivers are using electro-magnets to stop tachographs fitted to their vehicles working properly.
The Vehicle and Operator Service Agency (Vosa) recently targeted trucks in north Wales and fined seven drivers for having vehicles fitted with magnets.
They stop the tachographs to make it appear like they had stopped for breaks while the vehicles were still moving
The drivers were all fined between £400 and £600.
Vosa said convicted drivers could expect to appear before a Traffic Commissioner who has the power to revoke heavy goods vehicle licence for two years.
Tachographs record details about journeys made in individual lorries and are meant to ensure drivers observe rules restricting journey times and hours worked.
The seven drivers were caught during an 11-day exercise which involved vehicle checks.