By politics.co.uk staff
The postal strike began in the early hours this morning after last-ditch attempts to avert the action came to nothing.
Around 42,000 mail centres workers and drivers are striking today, with 80,000 delivery workers walking out tomorrow.
Thursday, Friday and Saturday next week will also see strike action.
Both sides in the dispute claimed to be ready to go to Acas, the arbitration service, although Royal Mail appears only ready to go once the strike is called off. Business secretary Peter Mandelson called for Acas to be used, while Downing Street called for talks to continue.
"It's regrettable the strike has started, but there's still an opportunity for a negotiated settlement," the prime minister's spokesman said.
Opponents of the strike are concerned about Christmas deliveries, the fortunes of small businesses - especially delivery businesses - which were just starting to emerge from the recession.
During PMQs yesterday David Cameron berated the prime minister for not forcing through his bill to part-privatise the Royal Mail. It was this weakness which contributed to an increased union militancy, Mr Cameron claimed.
Union officials made similar points yesterday, when they suggested Lord Mandelson's objection to the strike was a revenge tactic for the defeat of the bill.
"We're doing what we can," Mr Brown told the BBC today.
"This strike is self-defeating. It's essential everyone gets round the table."
Speaking on the Today programme this morning, shadow business secretary Ken Clarke insisted the Tories would try to push through part-privatisation if the party attains power, but that it would not break up the Royal Mail.