Talks have resumed between the Royal Mail and union leaders, as unofficial strikes spread across the country.

Post boxes in London are being sealed and special delivery services have been suspended, as the postal system grinds to a halt.

The prime minister and the trade secretary are both said to be concerned about the escalation in the wildcat action.

They have called on both sides in the dispute to continue active engagement in a bid to resolve the dispute.

Two thirds of the Capital's postal workers are now on strike, with industrial action spreading to areas including Milton Keynes, Coventry, Swindon, Stoke-on-Trent and parts of Berkshire, as part of a campaign to increase London weighting payments and working arrangements.

Companies have warned that they have lost tens of millions of pounds over the last ten days.

"What we're seeing is a concerted campaign, orchestrated by union activists, to blatantly force the company to increase the London weighting payments," chief executive Adam Crozier alleged.

However, the Communication Workers Union has denied the charge, arguing that the dispute was more concerned with the changes to working practices enforced in an attempt to improve Royal Mail's financial position.

"They are making demands on our members... which are about ripping out all of their local terms and conditions," said deputy general secretary Dave Ward.

The Department of Trade and Industry urged both sides to continue negotiations.

"No-one will benefit from strike action whether official or unofficial. It is damaging not only to the customers who rely on Royal Mail services but also to the company," said a spokeswoman.

"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."

Conservatives called on the government to open competition in the sector if strikes continue.

"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," said shadow trade secretary Tim Yeo.

"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."

Liberal Democrats have written to industry secretary Patricia Hewitt, urging her to intervene.

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

"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."