Ed Balls has called on the Government to reverse soaring fuel prices by abolishing the recent VAT rise in petrol.
In an interview with The Sunday Times, the shadow chancellor said the move would save 3p a litre and help the millions of middle-income families facing a "cost-of-living crisis".
Calling for immediate action, he said: "Filling up a family car now costs £65-£75. World oil prices are already very high, and the Chancellor has chosen, at this very moment, to raise fuel prices further, by pushing up VAT. I am urging him to reverse that increase."
The price of petrol is rising at the highest rate for 10 years, with some retailers already charging in excess of £1.40 a litre.
The crisis in North Africa and the Middle East has led to spiralling oil prices, which petrol retailers predict will filter down to the forecourts in the coming weeks. A 1p rise in fuel duty is also planned for the end of the month.
Experts believe the two factors could lead to an estimated 5p per litre increase by April 1.
Mr Balls claimed the Chancellor has no excuse not to ease the misery for motorists and warned that public anger could escalate "very quickly" if he refuses.
He continued: "Osborne will say that he can't make these sorts of decisions outside the Budget, but he recently announced a new bank tax at 7.22am on the (BBC Radio 4) Today programme. If he can have a mini-budget for banks, he can have a mini-budget for motorists."
Mr Balls did, however, acknowledge that the Government does not have total control over fuel prices, which are determined by world oil supply and international futures markets.
The Treasury said ministers were "actively examining every option" to relieve the burden on motorists.
One option being considered is the possibility of introducing a fuel duty stabiliser, whereby duty is cut when prices are high and raised when they fall.
But Mr Balls dismissed the idea as "completely flawed."
"It leads to instability and some perverse outcomes," he said.
The Treasury would lose around £700m a year by dropping VAT on fuel from the 20% rate, introduced last month, to 17.5%, according to independent calculations by House of Commons officials.
However, Mr Balls said this would be counterbalanced by around £800m it is expected to gain from Mr Osborne's new bank levy.