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UK energy minister, Malcolm Wicks, has branded 4x4s that produce large volumes of carbon dioxide "irresponsible", threatening higher taxes as a means of encouraging more people to switch to greener fuels.

At the same time, drivers of hybrid cars, which combine a petrol or diesel engine with an electric motor, will receive a stronger economic incentive, he said.

Revealing himself to be the proud owner of a Toyota Prius hybrid, he told the Times newspaper that measures "adopted so far, such as offering £1,000 grants to people buying a Prius", were no longer sufficient.

He said he would like to have bought a car manufactured in the UK or Europe, but was prevented from doing so by their failure to match Japanese models in terms of greener technology.

In addition to its Kyoto Protocol commitment to reduce carbon emissions to eight per cent below 1990 levels by 2012, the UK has pledged to reduce total emissions by 60 per cent by 2050.

Vehicle emissions will play a key part in this objective, including innovations in fuel.

"We are going to need a step change. We will have to ask 'is it environmentally responsible to be producing cars which are a serious part of the problem?' " commented Mr Wicks.

However, transport minister, Stephen Ladyman, also on the panel of the government's Energy Review, said that while greener choices would need to be made more financially attractive, he had personally considered the Prius for "about a millisecond".

Mr Ladyman drives an Alfa Romeo GT Diesel.