The Communication Workers Union has warned of new and more extensive strikes after meetings between its bosses and Royal Mail failed to reach an agreement.
The second phase of the nationwide postal strike is likely to continue into its second day this morning, with close to 44,000 members striking yesterday.
But CWU leader Billy Hayes said there was "every prospect"that industrial action would now be stepped up. The union is still considering whether to take legal action over Royal Mail's move to hire 30,000 agency workers to deal with the backlog of mail caused by the strike, as well as the Christmas rush.
Mr Hayes said yesterday: "We will be upping the dispute. We will not be scaling it down. There is every prospect that we will increase the action and we could be looking at longer strikes."
Picket lines were mounted outside mail centres across the UK at the start of three days of strikes which will cause huge disruption to mail deliveries for the second time this month. The union members walked out at 4am yesterday, after three days of intensive talks, chaired by the TUC general secretary Brendan Barber, failed to resolve the bitter dispute over jobs, pay and modernisation.
Picket lines formed outside Mount Pleasant sorting office in Central London, as well as in Leeds, Cardiff, Tyneside, Birmingham and Bristol.
Plymouth, Stockport and Stoke staff who redirect badly addressed mail were expected to walk out today and 77,000 delivery and collection staff will take action tomorrow.
Dave Ward, the union's deputy general secretary, said he had proposed a deal at 10am on Wednesday that he thought would end the dispute. He said that Royal Mail had not responded to the proposal by 6pm, so the union had no alternative but to let members know that the strikes would go ahead.
Royal Mail's managing director, Mark Higson, said they had been close to agreement on Tuesday evening but that on Wednesday morning the CWU had made fresh demands.
"Early yesterday the CWU came up with a fresh set of demands," he told the BBC. "We considered those and we were in fact at the TUC all day.
"But really, in an extraordinary action, the CWU then decided to carry on calling the strikes, while those talks were in progress."
Mr Ward said changes being planned by Royal Mail could lead to 60,000 jobs being lost and the remaining workforce being moved from full-time to part-time employment. Last night both parties were in fresh talks to avert further strikes, which sources suggest could restart this morning.
But the union leader also stressed that members want the conflict resolved before Christmas. "It would be a very difficult call for the union to take strike action at Christmas. We've never done that," he said.
Meanwhile, the Tories are reported by The Guardian to be backing the total privatisation of Royal Mail and hold the view that bidders are more likely to come forward if the dispute ends with the union forced to accept modernisation.