Even the Tories can't promise tax cuts now as Osborne delivers grim forecast on hoped-for relief

Mr Osborne, speaking as Tories gathered for their spring conference in Newcastle, downgraded his commitment on lower overall taxes to a long-term "ambition".

"I cannot make a promise at the election that there will be a big reduction in the tax burden or that there will be big tax cuts," he told BBC radio.

"I may well inherit the situation if the people vote Conservative ... where we have big borrowing figures to deal with."

Pointing out that Alistair Darling's forecasts were showing a £7billion leap in borrowing next year compared with forecasts of a few months ago, Mr Osborne said stability had to be his priority.

"My long-term commitment as a Conservative to people is that I will do everything I can to reduce their tax burden," he said.

"It is my ambition to reduce taxes on families and businesses. But the soundness of the public finances must come first."

Mr Osborne's tough line flew in the face of a survey showing overwhelming Tory backing for Thatcher-style tax cuts.

A poll of members by ConservativeHome.com found that 75 per cent wanted the party to pledge "tax cuts funded by tougher spending controls" while only 22 per cent said no.

In the past Mr Osborne appeared to pledge that he would "share the proceeds of future economic growth" between public spending increases and tax cuts.

However, senior Tories are now making clear that although public spending will continue to rise, taxes may not come down.

Instead, any short-term spare proceeds from growth may be used to pay off borrowings.

Mr Osborne appeared to have timed his comments as Labour was under fire to limit the damage.

Mr Darling faced new outrage yesterday amid claims that nine out of every 10 new cars will be hit hard by the Budget.

The figures appeared to blow apart the Government's claims that it was cracking down on gas-guzzlers to protect the environment.

It also emerged that a Porche or a Rolls Royce saloon will be relatively less hard-hit than family saloons and even some small cars.

From next year, owners of a Porsche Cayenne GTS or a Rolls Royce Phantom face a rise in Vehicle Excise Duty from £400 to £455 - an increase of 13.75 per cent.

But a modest Ford Focus 1.6 Duratec will attract a 59 per cent rise, from £170 to £270.

Buyers of a Vauxhall Astra 2.0i 16v or a Saab 9-3 MY2008 will pay twice as much - £430 instead of £210.

These changes occur from April 2009 when the number of road tax bands increases from seven to 13.

At the bottom of the categories is Band A for "green" cars emitting up to 100 grammes of carbon dioxide. This includes cars such as the electric G-Wiz.

At the top is the new Band M, which covers cars emitting more than 255 grammes of CO2. These vehicles, including the biggest 4X4s such as some Range Rovers, will be charged £440 each year.

Mr Darling said the motoring taxes were being imposed to "help motorists" to make more green choices.

But Treasury figures show the new showroom tax of up to £950 on the worst gas guzzlers in 2010 will rake in £1 billion for the Exchequer.

Nevertheless, a poll in The Times found public backing for some of Mr Darling's tax rises.

Some 68 per cent agreed that gas guzzlers should pay higher motoring taxes.