The government recently introduced an unpopular ‘retrospective’ £245 increase in tax for higher emission cars bought between 2001 and 2006 in an effort to encourage green motoring.
Cap believes that the drivers most affected by the move will be those who own automatic varieties of larger-engine cars which are up to seven years old.
A spokesman said: "We are not talking about luxury vehicles but ordinary family cars with a typical value of £3,000 or less.
"In years to come these new vehicle excise duty rates will come close to writing off the value of many."
The government is facing a backbench revolt over the proposed ‘poll tax on wheels’ and are alarmed that it will price many motorists off the roads.
Used car dealers will be equally worried by the way the tax changes will affect their business.