Proposals in the Labour partys leaked manifesto to hold the state pension age at 66 could see costs run to £300bn, according to analysis by Hargreaves Lansdown.
It shows that if current plans for the state pension age to rise from 66 to 67 between 2026 and 2028 are scrapped, it would affect approximately 11.7 million people and cost the government £93.6bn.
This is assuming that each person takes a flat pension rate of £8,000 a year, while if further plans to increase the age to 68 between 2044 and 2046 do not happen, it could affect 13 million people and cost the government £208bn.
"You have to put these numbers in context, the state pension costs around £100bn a year and these projections roll out over the next 30 years," Hargreaves Lansdown head of policy, Tom McPhail, said.
"So this analysis involves a total cost in today's money of around £3trn. In other words, these Labour proposals would increase the state pension costs by around 10%.
"If you want to freeze the state pension age at 66 and keep the changes cost neutral, you'd have to cut the state pension for every individual from £8,000 a year to around £7,200."
These calculations do not take into account the economic damage caused by people retiring earlier than they would have done under the current schedule, and assume around 650,000 people reach the state pension age every year.
Other pension plans revealed in the manifesto include:
Maintaining the triple lock
Legislation to ensure accrued state pension cannot be retrospectively changed
Guaranteed winter fuel allowance and free buss passes for everyone
Pensions protected for all UK citizens living overseas
Extending pension credit to compensate women born in the 1960s
An immediate review of mineworkers' pension scheme and British Coal Superannuation Scheme surplus sharing arrangements.
"Labour are making pensions a key battleground in the election," Hargreaves Lansdown senior pension analyst, Nathan Long said. "We already know the planned state pension age increases were due to save the government £30bn, so unwinding these will be very expensive.
"Plans to stop future tinkering impacting on accrued state pension may be appealing, but risk creating huge levels of complexity, and are probably best avoided."
Hargreaves Lansdown said the £300bn costs would have to be added to the national debt or found in some other way, with the Labour party yet to show how its proposals will be costed.
