Britain's Royal Mail has started sealing up nearly 15,000 London post boxes as unofficial strikes spread beyond the capital.
It aims to close the boxes by the weekend to end the build-up of undelivered mail, as talks with the Communication Workers' Union (CWU) enter their fourth day.
"Royal Mail will not be blackmailed," said Chief Executive Adam Crozier, accusing union activists in a statement of pressing postal workers in London to strike.
But the union was defiant: "Sealing up boxes shows a lack of commitment to the negotiations," said the CWU's Mr Chris Proctor.
Some households have been without mail for as long as ten days and many small businesses are beginning to suffer.
A spokesman for the London Chamber of Commerce said the action had cost British business £30 million sterling over the last 48 hours, adding that costs could spiral if it spread across the country.
The wildcat strikes, which started on October 17th in London, had spread to more than a dozen centres outside the capital on Friday.
Mr Proctor said the strikes had spread because Royal Mail had asked workers outside London to take on the work of their striking colleagues.
Royal Mail estimated that up to 20,000 workers were involved in the latest action, while the Union put the figure at around 25,000.