A row is simmering over the Budget announcement for increasing the road tax on diesel vehicles which cannot meet the proposed Real Driving Emissions Step 2.
The standard has not yet been agreed and does not come into effect until 2020 but the Chancellor Richard Hammond said new cars which do not meet it by April will cost their owners more.
He will move the cars up a tax band and that means nearly all drivers will face paying £500 more duty by 2021, according to Autocar magazine.
Recent sales of diesel cars have plunged 20% and the new ruling is expected to accelerate the decline as owners switch to petrol or electric vehicles.
It will also put pressure on car makers and dealers to trim prices if they want to sell the tax-battered cars .
Sales of affected models could lift before the April tax deadline but as it nears and afterwards it will become more of a buyers market and some models may be stuck on forecourts and holding lots as they fail to attract a sale. Bit pre-registrations are expected over coming months.
This could lead to essential discounting and dumping of models through fleets.