Workers at hundreds of Crown Post Offices are to strike on Easter Saturday in a dispute over closures, jobs and pay.
Members of the Communication Workers Union will walk out for 24 hours after backing industrial action by almost 9-1.
The union said up to 4,000 workers will be involved, at 373 Crown offices - the larger sites usually situated in high streets.
The action is in protest at plans to close or franchise more than 70 Crown offices, as well as over pay and jobs.
Picket lines will be mounted outside the offices, which handle around a fifth of Post Office business.
A number of Crown offices have already been franchised to firms such as WH Smith.
The union said Post Office staff have not received a pay rise since April 2011.
CWU deputy general secretary Dave Ward said: "With Post Office management refusing to negotiate, and with such a huge vote in favour of strike action, we have had no choice but to serve notice for strike action.
"The patience and loyalty of our Post Office members has been abused by Post Office management. This dispute is about protecting jobs and services as well as securing a fair pay increase for our members. We're saying to the public 'If you value your post office, support these workers on Easter Saturday and beyond'."
Andy Furey, CWU assistant secretary with responsibility for Post Office members, said: "Post Office management and their ill-conceived policies are out of touch with both its own staff and the communities it's meant to serve.
"The strike on Easter Saturday will be a celebration and defence of Post Office services, especially in the towns which face losing their Crown office.
"The Post Office says it is listening, but we say 'To who?' A huge majority of Crown office staff voted for strike action. That's a serious vote of no confidence in Post Office management.
"We want them to stop their flawed plans and instead recognise the immense value to customers of the Crown office network and its staff. If the Post Office continues to refuse to negotiate, this strike will be the first of many."
The Post Office said it was "disappointed" with the strike, adding that the CWU refused to understand the requirement for "critical change" across the Crown network which is losing £40 million a year.
The company said strike action would do nothing to change the need to drive forward its transformation strategy aimed at building a "profitable, modern and commercially viable" Crown network that is not reliant on government funding.
The Post Office said it had offered a series of cash payments of up to £3,400 to be paid before April 2015.
Contingency plans are in place to ensure that any disruption to customers is kept to an "absolute minimum".
Kevin Gilliland, network and sales director at the Post Office, said: "The CWU is ignoring the harsh commercial realities being faced by the Post Office and other retailers across the UK.
"Crown branches are currently losing £40 million per year and this is being subsidised by public money. This cannot continue.
"The Post Office is transforming its network to improve customer experience and in turn bring in new business.
"We are committed to the Post Office remaining a key part of UK high streets and our plans ensure this will happen. They mean that for 70 of our Crown branches - less than 1% of our network - we will be looking to partner with a suitable retailer, whilst at the same time we will invest £70 million in the remaining 300 Crown branches to modernise and grow services to ensure their long-term viability.
"Our pay offer remains unchanged. We want to make the first payment of up to £1,400 as quickly as possible. The unrealistic demands and call for strike action being made by the CWU will only delay how quickly this will be received into our people's pay packets."