A spokeswoman for CWU told Brand Republic it would be consulting all its members in the next week about Royal Mail's plans to alter its pension scheme, which could result in closing its final salary scheme for new employees and increasing the retirement age from 60 to 65.
CWU, which represents postal workers, and Unite, which acts for postal managers, said they would advise members to reject Royal Mail's proposed restructuring of its pension deals, and return the result of its ballot on March 26.
Were CWU and Unite members to reject Royal Mail's proposals, it is unlikely industrial action could be averted among the 150,000-strong membership organisations.
In a statement, Royal Mail said Unite and CWU had "agreed to support the changes" as part of wider agreements involving pay, modernisation and pension reform last autumn.
Last year, rolling strike action severely disrupted postal services in the UK between June and October, after union members rejected Royal Mail's proposals on pay, pensions and modernisation.
The latest threat of strike action comes after industry watchdog Postwatch criticised Royal Mail's performance in failing to meet first class delivery rate targets.
Postwatch warned 2007 and 2008 could prove to be "pivotal" in Royal Mail's future unless it could turn around its modernisation process and business performance.