Conservative leader David Cameron promised "significant incentives" to encourage motorists to boycott gasguzzling cars after it emerged he had spurned the offer of a free Jaguar.

A Tory government would slash carbon emissions from cars on British roads by almost a third to help the fight against global warming, he said.

The pledge came after it was revealed he had turned down the offer of a Birmingham-built Jaguar limousine in favour of a "greener" car made in Japan.

As Leader of the Opposition he is entitled to a free £50,000 XJ saloon, built in Castle Bromwich, in the same way as a Government Minister.

But instead, the Conservative Party will lease its own Lexus GS 450h, which is made in Tahara, in Japan.

The Lexus is considered m ore environmentally-friendly because it has a gas-electric hybrid engine.

A recent review of Government fleet concluded Ministers and the Conservative leader should receive a choice between a Jaguar or a Japanese-made Toyota Prius, which also has a hybrid engine.

T he decision followed intensive lobbying by West Midland MPs, who feared Jaguar might be ignored entirely. The Prius gives off carbon dioxide emissions of just 104 grammes per km, making it one of the cleanest cars available.

However the Lexus has emissions of 186 g/km, making it far closer to the diesel version of the XJ Saloon which has emissions of 214 g/km.

Mr Cameron's decision follows the announcement by Ming Campbell, the Liberal Democrat leader, that he is selling his "gas-guzzling" Jaguar to prove his green credentials.

Gordon Brown, the Chancellor, has also rejected Jaguar and ordered a Government Prius, as has Alistair Darling, the Transport Secretary.

But other senior politicians have stuck with West Midland cars. Shadow International Development Secretary Andrew Mitchell, MP for Sutton Coldfield, drives a Coventry-built Jaguar XK8, while Party Chairman Francis Maude drives a Rover 75.

Yesterday, Mr Cameron said he wanted to see the average emission level from new cars in the UK fall from about 170 grammes per kilometre today to a level of 100 g/km in 2022.

A working group led by former Environment Minister John Gummer would draw up detailed policies, he said.

"I want Britain to be at the forefront of international efforts to build a new generation of motor vehicles that are much less environmentally damaging," said Mr Cameron.

"Such a programme will include significant incentives to encourage the ownership of newer, greener vehicles."

It is thought these incentives could include reduced road tax, cut-price parking or exemptions from tolls and other road charges.

But Mr Cameron declined to say whether he would increase vehicle excise duty on gasguzzlers or impose green tax on air flights.

In his March Budget, Mr Brown announced a zero-rate road tax for low-emission cars, but none of the models which would be eligible is available in Britain.