LONDON - British postal workers called for further industrial action next week as they entered the second day Friday of a 48-hour strike over pay, conditions and modernisation.

Some 78,000 frontline delivery and collection postmen were set to strike on Friday, after an estimated 42,000 mail centre staff and drivers withdrew their labour on Thursday.

The strike, called by the Communication Workers Union (CWU), is causing widespread disruption to services at a time when state-owned Royal Mail is already losing business.

The union served notice of further strikes starting next Thursday, with details of how long they will last and which workers would be involved to be announced in the coming days.

The BBC reported that three days of strikes were to take place next week, starting Thursday.

This week's strikes are the first industrial action by postal workers since 2007.

Mark Higson, Royal Mail's managing director, said it was "appalling but sadly not surprising" that more strikes had been called.

Royal Mail managers insisted they were still open to talks, while Prime Minister Gordon Brown urged management and postal workers to get "round the table" to solve the dispute, warning it would cause long-term damage.

"If more and more customers leave the Royal Mail and more and more customers stop using the Royal Mail, then more jobs will be lost, so this is self-defeating," Brown said.

Business Secretary Peter Mandelson, who failed in a bid to partly privatise the service, urged union leaders not to politicise the dispute.

"I don't think trading insults in this situation helps resolve the dispute. Politicising or dramatising it is useless. People need to focus on what issues are dividing them," he said.

Union leaders blame the government and Royal Mail for trying to scupper an agreement and have offered "unconditional" talks at conciliation service Acas to break the deadlock.

CWU deputy general secretary Dave Ward said next week's strikes could be avoided.

"We have a week in which to reach an agreement. We are determined to get an agreement. We want Royal Mail to join us at Acas and work with us to achieve one."

Royal Mail has branded the strike "wholly unjustified", warning it would drive customers away from the business at a time when fewer people are sending letters and it faces growing competition from the private sector.

Royal Mail has said it intends to recruit up to 30,000 temporary staff to cope with the strike and the Christmas holiday rush.