Postal unions are threatening to ballot members for strike action if Royal Mail pushes through the closure of its defined benefits pension scheme.
The company has announced it will close the fund to new contributions at the end of March next year, "subject to trustee approval".
Unions say they will oppose any plans to close pension agreements made at the time of Royal Mail's privatisation in 2012.
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The Communication Workers' Union (CWU) says "a 50 year-old earning £25,000 a year and retiring at 65 would lose £4,392 a year, or more than £100,000 over the course of their retirement".
Ray Ellis, the union's acting deputy general secretary, says "any attempt by the company to impose change without agreement will be met with the strongest possible opposition, including a ballot for industrial action".
Unite officer Brian Scott added: "If we don't achieve a satisfactory outcome, we can't rule out an industrial action ballot on this issue."
The fund was already closed to new members in 2008. Since then new employees have been signed up to a defined contribution scheme that does not guarantee a set pension in retirement.
Defined benefit schemes were once the default option for company pension schemes. They guarantee a level of payout based on an employee's final salary, or in more recent years, average salary over their time with the business.
But as people are now living longer, the cost to firms of keeping these schemes sufficiently funded to meet future pension obligations has grown.
Royal Mail says its scheme, which is currently in surplus, costs £400m a year. The cost will rise to £1bn by 2018, according to the BBC.
In a statement it said: "We have concluded that there is no affordable solution to keeping the plan open in its current form."
The 90,000 workers still in the scheme will remain entitled to any benefits accrued so far, but future pension payments after the cut-off date will go into Royal Mail's defined contribution fund.
