Speaking before Nato launched airstrikes against Yugoslavia, Mr Blair said the raids would have the clear objective of reducing Yugoslav President Slobodan Milosevic's military capability to wage war against civilians in Kosovo.

But as the first US B-52 bombers flew out of RAF Fairford in Gloucestershire shortly before 10.45GMT, Mr Milosevic was told he could still avert military action by pulling back from the brink.

Robin Oakley in Berlin: "Tony Blair was clear it was time for action"

However, Mr Blair, speaking from the Berlin summit, said the Yugoslav leader had thwarted all previous attempts to reach a peaceful settlement.

"We must now act," he said.

Mr Blair acknowledged that British lives might be lost in the raids. But he warned that if nothing was done it was "an absolute certainty" that Mr Milosevic would continue to attack the people of Kosovo.

Blair: British lives could be lost

"This is a difficult decision but it's the right decision and we have to see it through all the way," he said.

Defence Secretary George Robertson is expected to make a Commons statement later on Wednesday about possible Nato air raids on Serbia.

The British embassy in the Serbian capital Belgrade has been closed as the threat of airstrikes in Kosovo moves closer.

Tony Blair defends Britain's readiness to commence military strikes

The staff left the building late on Tuesday evening to fly back to the UK.

Mr Robertson revealed that eight RAF Harriers equipped with laser-guided bombs and one mid-air refueling tanker plane are poised to join aircraft from other Nato countries in the event of any air strikes.

Defence Minister George Robertson: Averting humanitarian disaster is justified aim

HMS Splendid is also in the area ready to fire Tomahawk land attack missiles, he told the Commons Defence Committee.

Mr Robertson said he believed the aims of air strikes would be "inside international law and achievable".

He stressed that President Milosevic could stop the raids at any time by reducing his forces in Kosovo and signing up to the Western-brokered peace deal.

Ground troops ready

He also expressed confidence that Nato ground forces in Macedonia and Bosnia could defend themeselves if they were attacked by Yugoslavia.

George Robertson: Only military targets would be attacked

He said the Nato troops were on peace-keeping duties and any attack upon them would be "a gross violation of international law" and would result in "an immediate and considerable response from us".

But he reiterated that ground forces would not cross the border into Kosovo until there was a political settlement in place for them to police.

"We will not fight our way into Kosovo," he said.

Before departing for Berlin Mr Blair spelt out to MPs the extent of the human cost of the crisis in Kosovo.

He said the 15-month conflict had already seen 250,000 Albanians made homeless while another 170,000 had fled abroad.