Unite believes Royal Mail is planning to close its pension scheme to new employees, raise the normal retirement age from 60 to 65 and reduce the benefits paid out to members.
It has publicly warned Royal Mail it could call strike action, ahead of a meeting between Royal Mail and Unite representatives today.
Paul Reuter, Unite national officer, said: "We call upon Royal Mail to honour their commitment to preserve the past service benefits that have been built up and paid for by our members."
If Unite members strike it will worsen the current upheaval caused by the Communication Workers Union's strike action, which started in June.
The CWU last week called two 48-hour strikes for October 5-6 and October 8-9. It is in dispute with Royal Mail over pay, modernisation and pensions.