Postal workers from seven sorting offices are to take part in a 24-hour strike over the merger of some depots.
Mick Kavanagh, from the Communication Workers Union, said they would strike on 19 December as 500 jobs may be lost.
Workers from centres at Coventry, Crewe, Liverpool, Stockport, Bolton, Oldham and Oxford are set to take part.
Mr Kavanagh said there could be further action at the end of December and early January but the door was "still open" for talks with Royal Mail.
"It really is in Royal Mail's hands to get this sorted out," he said.
Among the firm's plans work from the mail centre in Weston, Crewe, and the Copperas Hill sorting office in Liverpool will be transferred to a new regional centre in Warrington.
Royal Mail, which announced its £50m regional plans at the start of August, said Copperas Hill was no longer viable in the face of falling mail volumes.
It said it wanted staff at both sites to work at the new centre, but unions said many workers would not be able to afford to travel or relocate.
Mr Kavanagh said 400 to 500 people could lose their jobs as a result of the plans.
He said 60% of CWU members who voted in a ballot were in favour of taking strike action and that the union "had no choice" but to take strike action because of the possible job losses.
He said he expected any industrial action would hit the postal service over the festive period.
"From our point of view there will be an effect and we'll do everything we can when we return to work to make sure we clear it [post] before Christmas."
On Thursday, Coventry MP Jim Cunningham asked the government to step in to try and head-off the potential strike.
He told the Commons he wanted ministers to arrange more talks between the Royal Mail, MPs and unions.