The Government is slapping a €900 tax hike on drivers who use their work vehicles for personal use.
Motorists who drive commercial 4 x 4s and small vans will be effected.
The AA has called the changes "silly" in a move set to hit small businesses in the pocket.
Under Environment Minister John Gormley's plan, owners of commercial 4 x 4s and small vans will have to sign a Goods Only Declaration in a Garda station.
They will have to state that they will not drive the vehicle for social, domestic or pleasure purposes.
If the gardaí subsequently catch them behind the wheel on a non-work related outing they will have to pay on average more than €1,200 motor tax rather than the reduced commercial rate of under €300.
The Environment Minister said the new rules close a tax loophole, but Fine Gael's Denis Naughten says it could be the final straw for businesses already on a tightrope.