A second union representing Royal Mail workers has warned it may ballot its members over industrial action.
Unite says it wants to protect workers' pension rights
Unite, which represents 12,000 managers, says the Royal Mail wants to close its pension scheme to current members and raise the retirement age.
Royal Mail denies any such decision has been made, though it does intend to close the scheme to new joiners.
Communication Workers Union members have already held a series of strikes in a pay and modernisation dispute.
Unite said as well as scrapping final salary pensions, it feared Royal Mail would increase its members' retirement age from 60 to 65.
"We call upon Royal Mail to honour its commitment to preserve the past service benefits that have been built up and paid for by our members," said Unite national officer Paul Reuter.
A Royal Mail spokesman said it would consult with staff before any pension changes came into effect.
"We have been clear that we will consult our people on proposals with the clear objective of protecting our existing people's pensions in a way that is both affordable and exposes no-one to unacceptable risks going forward."
CWU members are due to walk out again for 48 hours from 5 October and 8 October.
At the centre of the CWU's dispute is Royal Mail's 2.5% pay offer and modernisation plans.
The union claims the shake-up plans will put about 40,000 jobs at risk.
Royal Mail counters that without dramatic reforms it will not to able to survive in a liberalised mail market.