Firefighters were last night determined to battle on despite an uncompromising warning from the Prime Minister that they could not win their pay dispute.

Tony Blair gave a hastily-arranged news conference in Downing Street aimed at dispelling claims of mixed messages from ministers.

He warned that no extra money would be given to settle the row. Any rise above four per cent would have to be fully self-funding through modernisation.

The Government would not sign a "blank cheque" because of the damage it would cause to the economy and because it was unfair on other workers such as teachers, nurses, police and the military.

Mr Blair went out of his way to praise troops who were covering for firefighters on day four of the current eight-day-long strike and pointed out they were operating with just a third of the normal number of firemen and women.

He attacked the Fire Brigades Union (FBU) for its "unreasonable" pay claim, adding: "This is a strike they can't win. It would not be a defeat for the Government, it would be a defeat for the country."

The two sides in the dispute had been close to an agreement on a 16 per cent deal last week, but Mr Blair said that had not been costed and would have required more money from the Government.

FBU officials said Mr Blair's comments showed that the Government's position was hardening, making a settlement less likely.

The union has set two more eight-day-long strikes, starting on December 4 and 16 unless there is a breakthrough to the deadlocked dispute.

"It looks as though the dispute will go on until Christmas now," said one FBU official in London.

John Weaver, station commander at Manchester's Thompson Street station, said he was convinced the Government was now trying to turn all its efforts on destroying their support. "It's like setting a pack of wolves on a gerbil," he said. "We are not looking for that type of fight."

Mr Blair insisted he was not being "macho" in standing up to the union, but was being "completely straight and reasonable".

FBU general secretary Andy Gilchrist accused the Government of being "out of touch" with the public on the dispute. And he accused Mr Blair was "making a fool of himself" over claims that accepting the FBU's claim would destroy the economy.