Workers in hundreds of the country's biggest Post Offices are staging a fresh strike in a long-running row over jobs, pay and closures.
Members of the Communication Workers Union (CWU) at the so-called Crown offices will be taking industrial action for the third time in recent weeks.
The union is opposed to plans to close or franchise 76 of the Crown offices - the larger branches usually sited on high streets - warning that 800 jobs would be affected.
Campaigns have also been launched in areas affected by the proposals, with warnings that high streets struggling to cope with the economic downturn will be hit further if the main Post Office closes or moves into a supermarket or other local store.
The union is also in dispute over pay, saying its members in Crown offices have not received a rise for two years.
CWU national officer Andy Furey said: "The Post Office has admitted that it's got its sums wrong on the cost of our pay claim. They embellished the figure by over £6 million - effectively doubling the cost of what we've asked. Despite admitting this they still stubbornly refuse to negotiate.
"The Post Office's mistakes in relation to our pay claim calls into question the accuracy of other Post Office numbers - including the alleged size of the losses in the Crown network. This part of the network handles 20% of all Post Office business and delivers 40% of financial services sales, despite making up only 3% of the whole network. It really is the driver and heart of the network which is why we are so concerned about the closure and franchise plans."
Kevin Gilliland, Network and Sales Director at the Post Office, said: "We regret any inconvenience that may be caused by this needless strike action which affects less than 3% of the Post Office's 11,800 branches.
"The CWU is failing to accept the need to address the current loss of £40 million a year of public money across our Crown network. Through our critical modernisation plans we intend to turn this part of our business around and keep Post Office branches on high streets across the UK. We will invest £70 million in 300 Crown branches and are proposing to partner 70 branches with suitable retailers.
"These branches are not closing. Any move to a retailer's premises would offer access to the same range of Post Office products and high levels of customer service in a new modern branch. Our pay offer of three cash payments is extremely fair. The CWU's unrealistic demands are delaying our people from receiving the first payment of £1,400, which is ready to be paid into their pay packets."