Prime Minister Gordon Brown is 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.
