(writes through, new strike details)
By Avril Ormsby
LONDON, Oct 22 (Reuters) - Britain's postal union on Thursday called a fresh wave of national strikes for next week, threatening to paralyse a mail system already hit by stoppages and dealing a blow to Prime Minister Gordon Brown.
The Communication Workers Union (CWU) said it would extend this week's strikes held on Thursday and Friday, which it called over pay and job cuts, and to protest against modernisation plans for the postal service. The next wave starts on Oct. 29.
Royal Mail says modernisation is essential if the state-owned company is to fight off competition from more specialised delivery services and the Internet.
Brown said on Thursday that the strikes were self-defeating, and would lead to more job losses. He promised that the government would do everything it could to resolve the dispute.
"It's essential that everybody gets round the table. This strike is solvable, and I believe that management and the workforce can reach a solution to this and we should do this as quickly as possible," he told reporters.
The strike is potentially embarrassing for Brown and his Labour Party which receives a large chunk of its funding from the trade union movement.
About 42,000 mail centre staff and drivers walked out early on Thursday morning and 78,000 delivery and collection staff will strike on Friday this week.
Late on Thursday, the CWU said the strike would continue from Thursday of next week. Royal Mail responded by saying the decision was "appalling but sadly not surprising".
The opposition Conservative Party, which is well ahead in opinion polls before a national election due to be called by next June, attacked Brown's handling of the dispute.
Conservative leader David Cameron said the government's failure to go ahead with a planned part-privatisation of Royal Mail had galvanised union militancy.
Labour says it shelved the plan earlier this year because it could not find a buyer during the global economic crisis. But Brown also faced strong opposition from postal workers and within his own Labour Party.
Mail has already been held up by previous regional postal disputes. An estimated 150 million parcels and letters could become caught up in the latest backlog. (Additional reporting by Michael Holden and Matt Falloon; Editing by David Stamp)