LONDON - Talks between union leaders and the Royal Mail were resuming on Friday to try to avert a new wave of postal strikes next week.

Negotiations ended without agreement late on Thursday after a day of unofficial strikes by hundreds of postal workers in parts of London and Liverpool.

The wildcat action involved workers at 30 of the Royal Mail's 1,400 depots.

Members of the Communication Workers Union (CWU) are due to walk out again on Monday in the first of a series of one-day strikes over pay, pensions and shift changes.

That will compound delays and disruption caused by the last strike. Although deliveries resumed on Wednesday, postal workers were left with a backlog of more than 60 million undelivered letters.

Royal Mail has warned customers to expect more severe disruption if the planned action goes ahead.

In a statement, the union said it "remains committed to negotiating an agreement".

Prime Minister Gordon Brown has called on the CWU to end the strike, saying there was no justification for it.

Conservative Shadow Business and Enterprise Secretary Alan Duncan said the government should do more to end the dispute.

"Put simply, there is no urgency in trying to bring the postal strike to an end," he said.

Royal Mail is fighting private competition from rivals after losing its 350-year monopoly on postal services last year.

Its Chief Executive Adam Crozier says the company desperately needs to modernise to compete and that staff should be more flexible.

Union chiefs say that some working practices at Royal Mail amount to slavery.