His statement comes as Nato continues preparations for possible air strikes against Yugoslavia following crisis talks between US envoy Richard Holbrooke and the Yugoslav President Slobodan Milosevic.

Mr Blair's spokesman said Downing Street acknowledged that British lives could be lost in military action, but insisted any raids would be justified.

He said that Mr Milosevic appeared "hell-bent on provoking the humanitarian disaster that appears to be developing".

'We will act unless he acts'

Air strikes could begin while Mr Blair is abroad

The prime minister, in interviews with European journalists, has warned Mr Milosevic: "There is no doubting at all the seriousness of intent of the allied forced.

"We will act unless he acts.

"I cannot put it any more starkly than that and if he thinks that we lack the will to do that he is making a terrible mistake because we don't lack the will.

"There is no way now that unless Milosevic changes his position he will avoid action."

Humanitarian crisis

In his statement to the Commons, Mr Blair is expected to highlight the human scale of the crisis in Kosovo.

Air strikes would have to be jointly approved by Nato members

He will tell MPs that 250,000 Albanians have been made homeless by the 15-month conflict and another 170,000 have fled abroad.

The prime minister will add that 25,000 people have been forced from their homes since the withdrawal of OSCE observers last week.

Mr Blair is due to address the Commons at 1530 GMT before he flies to Berlin for a three-day EU summit in Berlin.

But his spokesman said there was no reason why UK forces could not go into action while Mr Blair was out of the country.

Any decision to begin air strikes would be taken by Nato Secretary-general Javier Solana, after consultation with national leaders.

Mr Blair's spokesman also ruled out the possibility that the thousands of British ground troops now massing in neighbouring Macedonia would be sent into Kosovo in the immediate future.