Postal workers have begun a second 48-hour strike as part of an ongoing row over pay, pensions and modernisation plans.
Royal Mail workers started a second walkout at 03:00 BST after marathon weekend talks between Royal Mail bosses and union leaders failed to bring about a resolution to the row.
At noon on Saturday some 130,000 members of the Communication Workers Union (CWU) ended a previous 48-hour postal strike.
The union is today planning to hold a mass rally in central London in support of the striking postal workers, with union leaders and former Labour MP Tony Benn among those set to speak at the event in Westminster.
However Royal Mail insisted on Friday that support was weakening for the latest round of strikes to hit the UK's postal service.
The company said that fewer of its workers were striking than in previous walkouts, meaning that it was still able to process mail and make deliveries to customers at reduced levels.
In response to the latest wave of strikes, Royal Mail has insisted that it "desperately needs to modernise and improve its efficiency" in the wake of growing competition from other postal service providers.
The company announced last week that the Unite union, which represents 12,000 Royal Mail managers, had accepted a 2.5 per cent pay rise that Royal Mail bosses proposed for staff.
But the CWU has rejected the pay deal and Royal Mail's modernisation plans, which it claims could lead to the loss of 40,000 jobs.
Reports suggest that the industrial action is likely to cost Royal Mail millions of pounds and could drive customers into the arms of competitors and to use other forms of communication.