Taxpayer-backed Royal Bank of Scotland has been accused of making its pension fund "unaffordable" after changing the terms of its main scheme.
Some 42,000 members of the bank's final salary pension scheme were told if they want to receive the full entitlement when they retire at 60, contributions must increase from zero to 5%.
Staff can still continue with zero contributions but must now work to 65 to receive full entitlement, RBS added.
Unite national officer David Fleming said: "With 28,000 workers receiving no pay rise this year, these changes will make access to the pension scheme unaffordable for many.
"The bank is attempting to push through these changes without any proper negotiations with the union. We will be campaigning against these plans and we demand that RBS gets around the table and negotiates with Unite."
RBS closed its final salary pension scheme to new staff in 2006 and introduced a cap to the scheme in 2009. The bank, which is 80% owned by the state, said the changes do not have an impact on contributions made by members to date.
RBS paid £785 million in bonuses last year, including £390 million for its 17,000 investment bankers.
While the total pot is 43% lower than the previous year, it follows a period in which the bank announced thousands of job cuts and its share price tumbled 50%.
An RBS spokesman said: "The changes announced today are essential to ensure that the group can afford to sustain the final salary pension scheme given the continued improvements in life expectancy.
"This also brings us in line with other UK banks and many other companies who have already increased the normal retirement age. We will be consulting on these proposals with Unite."