Keir Starmer has mounted a bid to block Rishi Sunak's pension tax giveaway to millionaires.
The Labour leader tabled an amendment that would halt the finance bill, which MPs must vote through to pass Jeremy Hunt's budget.
Labour had called for an increase in the tax-free pension allowance to be targeted only at the NHS - aiming to discourage senior doctors from retiring early.
Instead, the Chancellor introduced blanket changes to pension taxes - allowing the highest paid to build up an unlimited stash of tax-free savings away for their retirement.
Labour's amendment says the move "will cost around £1 billion a year and benefit only those with the biggest pension pots."
And it says the Budget "failed to set out an ambitious plan or growing the economy."
MPs will vote on the amendment later this evening, but because Rishi Sunak still commands a majority in the Commons, it is likely to fail.
The Treasury has claimed scrapping the tax-free limit was aimed at keeping doctors in the NHS, but that a blanket measure was the quickest way to achieve that goal.
But Britain’s highest earners - including longstanding MPs and Ministers, who benefit from extremely generous pensions - could also benefit from the move.
They include Mr Hunt himself and former Attorney General Geoffrey Cox, who pockets hundreds of thousands on top of his MP’s salary as a high flying barrister.
“I did 15 years in parliament and that used up 75% of my allowance,” Helen Goodman, Labour ’s former Work and Pensions minister told the Sunday Mirror last week.
“If I'd been a high paid minister for a long time, within that time period, my contribution would have been higher.”
Labour’s analysis of the plans found just 11 out of every 90,000 people in the UK will benefit from the tax giveaway - the equivalent of one football team in a packed Wembley Stadium.
Shadow Chief Secretary to the Treasury Pat McFadden said: “The vast majority of pension won’t benefit from the Government’s proposals because they will never get near a pension pot of more than £1 million."
