“This exemption applies to all vehicles including ones exempt from the payment of vehicle excise duty such as police vehicles.”

Today chief executive at the TaxPayers’ Alliance Matthew Elliott said: “Car taxes are sky-high and it’s wrong that the police expect us to pay ours and then allow their own to lapse with no consequences.

“If ordinary taxpayers were caught with untaxed cars they would incur a heavy fine, no questions asked.

“The police should lead by example and see that their legitimacy is undermined when they do not obey the laws they are supposed to enforce.”

Although police cars, trams, ambulances and lifeboat vehicles are exempt from actually paying road tax they must display an up-to-date licence.

The last available figures show a dramatic increase in the number of untaxed vehicles being clamped or towed away on Merseyside.

The DVLA said a total of 3,184 cars were seized in the 12 months to May 2008 – compared to 2,638 in the previous 12 months.

Cars can be seized for having no valid tax discs. And in some cases they are even crushed.

A Merseyside Police spokeswoman said: “It has been brought to the force's attention that three police vehicles were seen in Birkenhead displaying out-of-date tax discs.

“All police vehicles are exempt from the requirement to pay vehicle excise. However these vehicles should have been displaying a 'nil licence'.

“We will review our procedures to make sure all valid discs are properly displayed in the future.”