Downing Street and the Department of Trade and Industry have contacted both sides in the postal dispute to press for an end to the industrial action.

Tens of millions of letters remain undelivered in much of London and areas including Southend, Chelmsford, Dartford, Oxford and Milton Keynes.

A spokeswoman for the DTI said: "The government's position remains clear and constant - we urge the Royal Mail and the CWU to sit down together and reach a negotiated settlement on all outstanding issues. We understand these talks are ongoing.

"Both the unions and the management called for commercial freedom within the public sector and that is what we've given them. That is why the dispute is a matter for the unions and management to resolve.

"However, both the Royal Mail and the Department for Work and Pensions have contingencies in place to minimise the effect of action to ensure that anyone, including pensioners, entitled to benefits or other payments gets them."

Tim Yeo, shadow trade and industry secretary, said: "The further this strike spreads the greater the damage to Royal Mail, especially as there are now more alternatives for consumers to turn to than in previous years.

"Royal Mail is an organisation in difficulty and the longer this unrest continues, the harder the management will find it to provide a thriving and competitive mail distribution service.

"The government should make clear that if a strike continues, the restrictions that stop other organisations from providing letter delivery services should be lifted for the duration of the action in order to minimise disruption to consumers."

Malcolm Bruce, trade and industry spokesman, said: "Royal Mail is state owned and provides a service which is part of the country's essential national infrastructure.

"If Royal Mail is to continue to have the benefit of a protected monopoly to provide a universal mail service both management and workforce should accept their responsibility to resolve these disputes while maintaining the service.

"Failure to do so will accelerate the decline of Royal Mail as business goes elsewhere and undermine its ability to deliver a valid universal service at all.

"Accusations between management and employees appear to becoming increasingly bitter clearly suggesting that urgent arbitration is required.

"Legislation should be introduced to require compulsory arbitration in cases where the critical national infrastructure of the country or essential public services are threatened.

"I therefore urge you to take the action necessary to get the mail back on track and assure you the Liberal Democrats will support constructive initiatives in that direction."

Forum Response: Federation of Small Businesses

John Walker, policy chairman, said: "Small firms still rely on the post for vital documents such as invoices, contracts, legal forms and most importantly cheques. Royal Mail processes a million cheques each day, many of which are now caught up in the backlog.

"A delay of a couple of days in banking a cheque from a major customer may mean that a small business is unable to settle its own bills or pay wages to staff.

"The wildcat nature of these strikes has made them particularly damaging as it has prevented businesses from planning ahead or seeking alternatives. Despite email, the business community is still paper driven, and most of that paper arrives in the post.

"Most small business owners speak to their postal workers every couple of days and generally have a good relationship with them. But these strikes are having a devastating effect on small firms.

Forum Response: Institute of Directors

A spokesman for the IoD told ePolitix.com: "The current unofficial postal strikes sweeping across Britain are a serious blow for business.

"Smaller companies in particular who cannot not afford couriers will be suffering greatly. The longer these strikes go on the more harm will be done not just to Royal Mail but to the UK economy in general."