Prime Minister Gordon Brown was today facing the prospect of another damaging rebellion by Labour MPs over tax.

Backbenchers urged Chancellor Alistair Darling to re-think plans announced in the Budget for big increases in vehicle excise duty on "gas guzzling" cars.

Although vehicles bought before 2001 are exempt, MPs are concerned that the some owners who bought bigger cars in the past not realising the changes were on the way, could be faced with increases of up to £200.

More than 30 Labour backbenchers have now signed a Commons motion urging ministers to re-think the proposals before the new rates come into force.

Labour MP Ronnie Campbell, who tabled the motion, warned that the impact of the increases could be similar to scrapping the 10p tax rate, which led to Mr Darling's £2.7 billion climbdown in an emergency "mini budget" earlier this month.

"It is unfair (on) people who bought their cars a few years ago not knowing that the Government were going to put this road tax on," he told BBC Radio 4's The World at One.

"When people get their road tax letter through the door next year and find they have got an extra £200 to pay - well, I don't have to say any more, do I? The motorist is taking the brunt again."

Mr Campbell, who is due to meet Mr Darling when MPs return to Westminster after this week's Whitsun break, also called on Mr Darling to drop the planned 2p increase in fuel duty due in October.

He said that with rising world oil prices pushing up prices at the pumps, another increase would be too much for many families.

"I think people just at this moment can't afford it. They really are finding the pinch," he said.

"You have got to look at both sides of the argument, but at the minute I would be thinking that the Chancellor should take the 2p off."

The demands represent another headache for Mr Darling and Mr Brown at a time when the Prime Minister is already politically weakened by the debacle over the 10p tax rate and Labour's disastrous performance in the council elections and the Crewe and Nantwich by-election.

At the same time, the £2.7 billion emergency compensation package for people hit by the abolition of the 10p rate has left the Chancellor virtually no room for manoeuvre with the public finances.

Ministers were today talking tough. Environment Minister Joan Ruddock dismissed suggestions that the increases in vehicle duty had come as a bolt from the blue.

At the same time she said that the Government could not afford to abandon its environmental agenda for tackling climate change.

"I think the direction that we have all been going in has been clear to people for some time," she told The World at One.

"These things are all a matter of balance. What we cannot afford to do is to lose sight of the environmental agenda because this is everybody's future. This is the future of the planet.

"We have got to have measures - we can be smarter about them - but anybody who suggests that this environmental agenda can go away is talking nonsense."

Labour MP for Wolverhampton South West Rob Marris warned that there was "a significant risk" that car tax could cause anger on the scale of the 10p tax rate abolition.

Mr Marris, a parliamentary private secretary in the Northern Ireland Office, said that it was "undesirable" that hikes in VED should be applied to cars as much as seven years old.

He said he was hoping to meet Mr Darling when Parliament returns next week and would urge him to rethink the changes, which were announced in the Budget but will not come into effect until next year's Finance Bill.

Mr Marris told BBC Radio 4's PM: "There are 30 million motorists in the UK. Not all of them will be adversely affected - some of them will gain - but there will be millions of people adversely affected.

"It is not about changing future behaviour, because of course these cars already exist in the fleet.

"Therefore it is retrospective taxation. I am in favour of prospective green taxes to change people's decisions when they buy a car, but to tax them heavily on a car when it was bought seven years ago doesn't seem a good way to go and it will discredit green taxes."

He warned that the changes in VED could hit drivers of ordinary family cars.

"Mondeo man could be affected," said Mr Marris. "Millions of people will be affected... Medium-sized family cars, depending on what sort of engine they have and what sort of emissions they have, could be hit very hard.

"I shall be urging Alistair Darling to have a rethink on this issue... There's still time to have discussions on this and I hope these discussions will be fruitful."

A Treasury spokesman said: "The reforms to VED aim to ensure that people gain financially by choosing the car that pollutes the least.

"For 24 out of the 30 most popular models bought in 2006, drivers will generally pay lower rates of VED than they do now, and from 2010 cars that pollute less than 130g/km will pay no VED.

"The higher first year rates will send a signal to people at the point when they purchase a new car that they can save money by choosing a lower emitting car."