Dave Ward, deputy general secretary of the CWU, threatened strikes unless Royal Mail opens negotiations over the issue.
Ward said: "With such overwhelmingly opposition to their plans, if Royal Mail can't find an acceptable solution with the CWU, industrial action will be inevitable."
The CWU claims it is talking to the Royal Mail managers' union Unite about co-ordinating industrial action in the event Royal Mail refuses to enter into "meaningful discussions".
Royal Mail managers also rejected the pension changes two weeks ago.
Royal Mail closed its final salary pension scheme to new members after March 31. The changes also mean that benefits earned after April 1 2010 cannot be taken until employees reach the age of 65, as opposed to the current threshold of 60.