Gordon Brown told MPs that postal staff should go back to work, as the Government took a hard line against the Communication Workers Union (CWU) in its long-running dispute over pay, jobs and pensions.

His message was reinforced by Business Secretary John Hutton, who toured radio and TV studios to say that the union should accept the "perfectly fair and reasonable" offer which was on the table and return to work. He warned that the row will cause "irreparable damage" to the Royal Mail if it goes on.

TUC general secretary Brendan Barber said: "I am disappointed that the Prime Minister should intervene in this way, at such a delicate stage in the negotiations. This will not help find a sustainable resolution to the dispute."

The row blew up as postal workers in more than 20 delivery offices took unofficial industrial action just hours after ending a 48-hour strike. Workers in several parts of the UK, including Glasgow, Liverpool and parts of London, took action in protest at changes to their hours.

Some protested that they arrived for a shift starting at 5am but were told that the Royal Mail had implemented changes to shifts so they could not start until 6am.

Up to 130,000 union members were due to return to work on Wednesday morning after a second 48-hour strike, but unofficial industrial action broke out at the start of the morning shift. By mid-morning, postal workers were still taking unofficial action in some parts of London and Liverpool after returning to work in Glasgow. Overall, there was action in around 24 of the 1,400 delivery offices nationwide.

"The overwhelming majority of our people are now back at work clearing a backlog of mail which is equivalent to less than a day's normal postbag," said a Royal Mail spokesman.

The company said it had given notice over a period of months of changes to the start times of some workers because of new legislation which will restrict the speed of mail vehicles.

The union has announced a fresh series of strikes which will hit deliveries next week. It said in a statement: "CWU remains resolute in seeking an acceptable negotiated settlement. Elements of Royal Mail's proposal remain unacceptable and we hope to resolve these outstanding areas through negotiation."