Post Office workers have announced their fifth strike since Easter as they warned bosses not to "plough blindly ahead" with deeply unpopular plans to close 70 big high street branches.
The Communication Workers Union (CWU) said bosses had failed to listen to their concerns about the plans to franchise out 70 of the Post Office's 370 "crown" branches to retailers and the union had "little choice" but to walk out again on Tuesday 28 May.
Dave Ward, deputy general secretary of the CWU, said there was "massive public opposition" to the closure plans that will "rip the heart out" of the Post Office network.
"There is growing unrest within the Post Office and it's time that management responded to workers' concerns," he said. "They cannot continue to plough blindly ahead with deeply unpopular and unnecessary plans."
Ward said workers had been touched by the "fantastic support from customers, politicians and local communities".
"We're fighting to protect Post Office jobs and services, which are clearly valued," he said.
The union's 4,000 crown post office members voted nine-to-one in favour of the full-day strike the day after the second May bank holiday. Staff have already held four strikes against the franchising plan since Easter Saturday, including one on the Tuesday after the first May bank holiday.
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The Post Office, which is state-owned and receiving £1.3bn of taxpayer subsidy, said franchising out 70 of the crown branches was essential to securing the future of the service.
Kevin Gilliland, the Post Office's sales director, has said the crown network of the biggest post offices across the country is "losing £40m a year of public money" and "must change the way they operate to ensure long term viability".
The CWU claims up to 800 Post Office jobs will be affected and staff are likely to receive inferior pay, pension and working conditions if they are transferred to retailers' books.
The Post Office said it had offered crown network staff an "extremely fair" £3,400 staged payment in recompense. The payment is worth about one sixth of an average Post Office employee's annual salary.
The union said it had managed to secure a £100 payment for staff at its last meeting with management, but "no progress was made on bigger issues" of branch closures and a pay deal.
Crown post offices represent just 3% of the nationwide network of 11,800 branches, but are generally the largest offices and handle 20% of all transactions and 40% of financial services sales, a key growth area for the Post Office.
Six years ago 85 crown outlets were closed in the Post Office's last turnaround plan and 70 were moved into WH Smith stores.
The Post Office has already received 150 expressions of interest from retailers keen to run post office services in their stores. Subject to a public consultation the crown branches will be transferred to the winning retailers before April 2015.
The Post Office was, along with Royal Mail, part of the General Post Office until that was abolished in 1969. The Post Office became independent of Royal Mail, which the government is planning to privatise amid protests by workers, in April last year.