Postboxes in London will be sealed today because of a series of wildcat strikes by thousands of postal workers which has crippled deliveries and led to a mounting backlog of letters.

The unofficial action has brought postal services to a standstill in many parts of London and spread yesterday to parts of the country including Coventry and Warrington.

The Communication Workers Union said an estimated 25,000 workers had now walked out because of a row which started almost two weeks ago when a driver in west London was suspended.

The union's general secretary, Billy Hayes, and his deputy, Dave Ward, unexpectedly turned up at the Royal Mail's London headquarters yesterday and held talks with its chief executive, Adam Crozier. Further talks took place last night and are expected to resume today between regional officials of the union and managers but there was little sign of a breakthrough in the dispute, which is costing the Royal Mail millions of pounds and causing huge problems for businesses.

Workers are now taking unofficial strike action in Essex, Maidstone, Milton Keynes, Warrington and Coventry.

The Royal Mail will today begin sealing some of London's 20,000 postboxes for the first time in several years. Mail will be emptied from the boxes before they are taped up and taken to sorting offices to join bags of other undelivered post.

Mr Crozier has accused union activists of prolonging the dispute and said they were trying to cajole workers across the country into taking industrial action. He insisted that an increase in the London weighting allowance, which is at the centre of the dispute, would not be granted.

The union has accused managers of intimidating workers. Business leaders have urged the two sides to reach an agreementbecause of the damage that the lack of normal postal services is doing to companies across the UK.