Thousands more Royal Mail workers could be balloted over possible strike action, it emerged today.
Unite, the union which represents some 12,000 Royal Mail managers, is warning the postal service provider that it could face industrial action by its members over expected changes to workers' pensions.
The warning comes after the Communication Workers Union (CWU) announced last week that its members employed by the postal service will hold further strikes next month, in an ongoing pay and jobs dispute with Royal Mail.
In a separate row Unite has described an expected change to Royal Mail's pension scheme as "the great mail robbery".
The union, which is to meet with Royal Mail bosses today to discuss its concerns, claims that the organisation is planning to "bank" current pension benefits in a move it says could cost members of the scheme up to £1.5 billion.
Unite, which expects Royal Mail to inform employees of changes to its pension provisions during the next week, also believes that the scheme could be closed to new employees.
In addition, union leaders claim that existing final salary schemes could be replaced and the normal retirement age for postal workers raised to 65 in 2010.
Unite national officer Paul Reuter 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.
"Failure by Royal Mail to respond positively will leave Unite with no option other than to ballot its' members for strike action," he added.
A Royal Mail spokesman said: "No decisions have yet been taken as 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."
Last week the CWU described Royal Mail's proposed settlement on pay and business charges as "draconian and destructive".
The union has previously claimed that Royal Mail's modernisation plans will result in the loss of up to 40,000 jobs, but postal service bosses have insisted that the changes are necessary to maintain a viable business in the future.