The Royal Mail today began sealing post boxes in an attempt to stem a mounting backlog of letters as more workers joined an unofficial postal strike.
Workers in Swindon, Slough and Gravesend today joined the wildcat action, taking the number of postmen and women on strike across the country to more than 20,000.
The government has been urged to intervene in the dispute, which is costing the Royal Mail millions of pounds and causing huge problems for businesses.
Talks between the Royal Mail and Communication Workers Union (CWU) officials will resume later today. They were adjourned last night, with little sign of any breakthrough.
Today, the Royal Mail began sealing some of London's 20,000 post boxes for the first time in several years. Most of the capital's post boxes are expected to be affected by early next week if the dispute continues.
Mail will be emptied from the boxes before they are taped up, and will be taken to sorting offices to join bags of other undelivered post.
The industrial action, which began in west London two weeks ago, has been spreading every day, leading to a mounting backlog of post.
Workers in Maidstone, Essex, Milton Keynes, Coventry, Oxford, Portsmouth, Warrington, Rugby, and at Stansted airport, are also taking unofficial strike action.
Malcolm Bruce, the Liberal Democrat trade and industry spokesman, said that "urgent arbitration" was needed to resolve the escalating row.
Mr Bruce has written to the trade and industry secretary, Patricia Hewitt, calling on her to act to end the "deeply damaging" dispute.
"Accusations between management and employees appear to becoming increasingly bitter, clearly suggesting that urgent arbitration is required," he said.
"Legislation should be introduced to require compulsory arbitration in cases where the critical national infrastructure of the country or essential public services are threatened."
