Workers in hundreds of Crown post offices staged a fresh strike today in a long-running row over jobs, pay and closures.
Members of the Communication Workers Union in England, Wales, Northern Ireland and most of Scotland walked out, and will be followed by a strike in a handful of branches in Scotland tomorrow.
The union said the stoppage, the 12th round of industrial action since Easter, was well supported, warning that the dispute would not go away.
Staff are also taking other forms of action including a sales ban on financial products and services.
The Post Office said almost nine out of 10 of the Crown offices were open despite the strike.
The union is opposed to plans to franchise or close 75 Crown offices, the larger sites usually found on high streets.
Dave Ward, CWU deputy general secretary, said: "'Today's strike is extremely well supported and remains as strong as it was on the first day of the 12 strike days so far taken. The Post Office wants to downgrade the network and reduce services but post office workers continue to defend jobs, services and fight for a pay increase'.
'The company thinks this dispute will simply fade away but today's support for strike action proves that CWU members continue to be resolute."
Kevin Gilliland, network and sales director at the Post Office, said: "This action does not affect over 97% of the Post Office network and 87% of the around 370 Crown branches are open for business today.
"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 proactive sale of our growing range of products and services is a vital part of transforming the network and meeting our objective of getting the Crown branches to break even by March 2015.
"We would like to reassure customers that they will be able to continue to access all services from our branches."
A Department for Business spokesman said: "The Government is fully committed to modernising the post office network and to safeguarding its future. Eliminating the heavy losses from the 370 branches of the Crown post office network by March 2015 is a key element of the Post Office's strategy to make the network sustainable in the long term.
"The Government supports the Post Office delivering that strategy, which is why we are investing £1.34 billion over the duration of this Parliament to maintain and modernise the network, helping the Post Office to compete in a changing retail market.
"The Government does not play any role in pay negotiations between Post Office Ltd and its staff."
A Royal Mail spokesman said: "Royal Mail and Post Office are sister companies. Our mutual commercial success is best served by Royal Mail and Post Office working closely together.
"Post Office is Royal Mail's biggest customer and we have a long-term agreement in place.
"A stronger Royal Mail will mean a stronger Post Office."
A Business Department spokesman said: "This Government is providing £1.34 billion to both maintain and modernise the post office network.
"This means that there will be no programme of post office closures. Instead we have committed to maintain a network of at least 11,500 branches and to transform thousands of branches.
"The Post Office is making good progress towards becoming a front office for Government, winning every contract it has bid for in the last two years. It was recently announced that the Passport Office has signed a seven-year extension with the Post Office.
"In addition, the Post Office is continuing to introduce new financial products and services, including a range of current accounts that are being piloted ahead of an anticipated national rollout in 2014."