Rishi Sunak must drop his threat to make millions of people work longer, voters will demand tomorrow.
Ministers will decide after the next general election when to raise the state pension age to 68.
Campaigners on Tuesday will call on the PM to axe completely proposals to force people to retire later.
People currently in their late 40s or early 50s face being hit by ministers delaying their retirement date.
MPs will join workers and pension campaigners on College Green to fire a shot across the bow of the government.
Caren Evans, Unite national officer for retired members, said: “Unless Rishi Sunak completely rules out raising the State Pension I believe the odds are firmly in favour of a nasty surprise for middle aged workers after the election if the Tories win.
“After 13 years of austerity and public sector cuts the Tories can’t be trusted with your State Pension. That’s why we are demanding rock solid assurances from the Prime Minister ruling out any further rises.”
Jan Shortt, general secretary of the National Pensioners Convention said: “Workers of today are the pensioners of tomorrow. Raising the retirement age when the health of the nation is at its worst is unacceptable.
“It would show a lack of commitment by Government to tackle issues that impact adversely on older people.”
The current retirement age is 66 and will increase to 67 in 2028 before the next scheduled rise to 68 is due in 2046.
However, ministers have been considering bringing forward the change by a decade, affecting those who are between 46 and 54-years-old today.
Work and Pensions Secretary Mel Stride announced earlier this year that a decision will be made within two years of the next general election.
Ministers had ordered a review on raising the retirement age more quickly, but postponed a decision amid warnings of an electoral revolt.
France has faced months of riots after Emmanual Macron forced through pension reforms without a vote in parliament.
Under his changes, the legal retirement age there will rise from 62 to 64.
Workers today marched across France in protest at Labour Day rallies, with police firing tear gas in Paris and the western city of Nantes.
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