Staff in hundreds of Crown Post Offices will stage a fresh strike on November 20 in a long-running dispute over jobs, pay and branch closures, the Communication Workers Union said.
A Unite official has defended the union's decision for Post Office managers to go on strike.
Brian Scott said: "While we understand that the industrial action will cause inconvenience to the public, the ball is in the management's court - as it has been for the last 16 months - to reach a fair and equitable settlement.
"Unite is ready to have talks at anytime with the Post Office to resolve this matter, but the management has taken a very hardline stance against our members who have seen their living standards seriously eroded."
Hundreds of Crown Post Office managers will go on strike today in a row over pay.
Unite warned that the action will hit services at the larger branches, usually sited on high streets.
The union, which represents 900 managers, said its members have not received a pay rise since June 2011 and accused the Post Office of "dragging its feet."
Unite members will not undertake any overtime, will refuse to work any additional hours or do work not proper to their role in the organisation.
They will also refuse to travel outside of their working time, nor will they work in a location other than their normal place of employment.
This is an important day for Unite members in the Post Office.
They are keen to impress upon their employer their dissatisfaction with the failure to reach a pay agreement with the union and see this an opportunity to protest.
Post Office managers will stage their first industrial action in 34 years in a dispute over pay.
The Unite union, which represents 900 managers, said its members will ban overtime and work to rule for 24 hours.
The union accused the Post Office of "dragging its feet" for 16 months over a pay deal and warned of further industrial action if the dispute is not resolved.
Post Office managers are to ban overtime and work to rule on Friday in a row over pay - the first time they have taken industrial action in 34 years, said the Unite union.
Kevin Gilliland, network and sales director at the Post Office, said the Communication Workers Union's (CWU) decision to strike will "cause disruption to customers."
The CWU are calling for an above-inflation pay agreement when last year our overall performance was a loss of £116 million.
We must reverse this loss of public money. Strike action can only cost our people money, cause disruption to customers and threaten our plans to turn the business around and keep our branches on high streets and in city centres across the UK.
The CWU's call for its members to stop proactively introducing financial services to customers is extremely irresponsible.
The Communication Workers Union deputy general secretary Dave Ward said post workers were striking today due to the "reckless attitude of the post office".
If the Post Office thinks this dispute will simply fade away they are sadly mistaken. Our members are fiercely opposed to the company's plans to close and franchise offices, slash jobs and impose a pay freeze.
The company's plans are to downgrade the network, reduce services to local communities and hit jobs in the network. There is no mandate for this course of action and customers across the country are appalled at the reckless attitude of the post office towards these public services.
Mr Ward added that the strike, which will affect up to 4,000 staff in 372 offices today, could carry on into the Christmas period.
The strike and the continuing sales ban activity will continue into the Christmas period unless management see sense and negotiate a fair deal for Crown office staff.
Staff at hundreds of Crown post offices will today stage a fresh strike in a row over jobs, pay and closures.
Members of the Communication Workers Union in England, Wales, Northern Ireland and most of Scotland will walk out in the 12th round of industrial action since Easter.
The union is opposed to plans to franchise or close 75 Crown offices, the larger sites usually found on high streets.
Some staff will be taking other forms of action including a sales ban on financial products and services.
The strike today will be followed by action in a handful of branches in Scotland tomorrow.
The postal workers' union has threatened to continue strike action into the Christmas period unless they can reach a "fair" settlement with Post Office management.
Dave Ward, CWU deputy general secretary, said:
If the Post Office thinks this dispute will simply fade away they are sadly mistaken. The company's plans are to downgrade the network, reduce services to local communities and hit jobs in the network.
Next week's strike and the continuing sales ban activity will continue into the Christmas period unless management see sense and negotiate a fair deal for Crown office staff.
The union said next week's strike will affect up to 4,000 staff in 372 offices.