Hundreds of Post Office managers are to strike on Saturday.
Unite said the 24-hour walkout will disrupt overseas Christmas mail.
It’s set to affect Crown post offices – 300 of the network’s 11,600 branches – such as the Northumberland Street Post Office in Huddersfield.
Around 720 managers are taking action in a long-running pensions’ dispute. They’re joining a strike by members of the Communication Workers Union, which is in dispute over jobs, the franchising of Crown Post Offices plus the closure of a final salary pension scheme.
Brian Scott of Unite said: “We believe that Saturday is the day when most people will be dispatching their cards and parcels to their relatives and friends abroad.
“We are taking this action because the management refuses to talk in a constructive manner about the pension scheme which is currently in surplus to more than £143 million.
“This is the retirement income of our members which is at stake and we are not going to stand idly by and let them lose thousands of pounds when they retire.”
Post Office spokesman Mark Davies said they are “sorry the unions are attempting to disrupt services for people sending parcels and cards to loved ones in the run-up to Christmas.”
Saturday is the last standard delivery postal date to Africa and the Middle East.