Postal workers will mount picket lines across the country on Thursday when they launch a 24-hour strike in disputes over jobs, branch closures and pensions.
Members of the Communication Workers Union and Unite are taking industrial action in protest at the closure of Crown offices, job cuts and changes to the pension scheme.
The Post Office said most of its 11,600 branches will remain open despite the walkout.
Dave Ward, general secretary of the CWU said the Post Office was at “crisis point”, urging the Government to stop the “cycle of closures job losses and attacks on workers’ terms and conditions.”
Terry Pullinger, the CWU’s deputy general secretary, said it was now “do or die” for postal workers and post offices.
He said the Government and the company described the Post Office as a success story.
Mr Pullinger added: “Their idea of success is to sell Crown office buildings, put the money in the Government’s coffers, then outsource or franchise the service by sticking it in the back of a WH Smith, redeploy decent, union-negotiated jobs, and replace them with minimum wage, zero hours contracts.
“The Government has now closed the defined benefit pension scheme, probably the best funded in the country.”
Ivan Monckton of Unite said the Government had promised to make post offices the front office of its services, but it was more like the “outside dustbin area.”
He accused the Government of presiding over an “ill-managed decline” of the Post Office.
Kevin Gilliland, the Post Office’s network and sales director, said: “We can reassure customers that most Post Office branches will be open for business as usual.
“We will also work hard to minimise any disruption to customers in our 300 directly-managed branches should they be affected by strike action.
“Our contingency plans also ensure that all of the network can operate normally for our customers with sufficient cash and stock.”
Mr Gilliland said the Post Office regretted the unions’ decision to take industrial action.
“The Post Office is a national institution that is successfully adapting to change and we are safeguarding the future of the network,” he said.
“More than 99% of people in the UK live within three miles of a Post Office branch and we are committed to secure that level of service for the future.
“In just over three years we have modernised over 6,000 branches, adding more than 200,000 extra opening hours each week and becoming the largest UK retail network open on Sundays.
“We have also halved our losses and we are making steady progress to reduce costs to the taxpayer.”