Mr Blair was flying to Washington on Wednesday for Nato's 50th anniversary summit, which will be dominated by events in Kosovo.

Leading British politicians now insist Serbian President Slobodan Milosevic cannot be allowed to veto any ground force.

Speaking in Prime Minister's Questions on Wednesday afternoon, Mr Blair repeated the Alliance's insistence it was determined to succeed against President Milosevic.

Answering a question from opposition leader William Hague, Mr Blair said: "The difficulties of a land force invasion of Kosovo against an un-degraded Serb military machine are formidable and they have been all the way through.

Tony Blair: Land invasion force faces "formidable difficulties"

"I've also said, and repeat now, that of course Milosevic does not have a veto on Nato action.

"All options are always kept under review - that is sensible for us to do."

Liberal Democrat Deputy Leader Alan Beith asked the prime minister whether he accepted that ground troops may have to secure peace in Kosovo when Serbian forces have been sufficiently weakened by air strikes.

The Kosovo conflict will dominate Nato's summit

The prime minister said: "I don't have anything to add to what I've ready said about the difficulties that would face a land invasion force with an un-degraded and undiminished Serb military machine.

"But I do repeat: Milosevic cannot have a veto on Nato action."

Mr Blair and Foreign Secretary Robin Cook have signalled Nato ground troops could be sent into Kosovo without a peace deal being reached with President Milosevic.

Nicholas Witchell reports on the latest Nato strikes, 4 weeks since the start of the bombing

The UK and US governments had previously ruled out sending ground forces into a hostile environment.

Defence Secretary George Robertson echoed those sentiments in Wednesday's Ministry of Defence briefing in London.

"More land forces have been sent to Macedonia in readiness to deploying into Kosovo once the air strikes have done their job, so that the Kosovo people can return to their homes.

"Milosevic will not have a veto."

Nato troops stationed at the borders of Kosovo are currently helping the humanitarian effort. Far greater numbers would need to be deployed to begin a ground invasion.

The UK, the US and France accept they should plan for the deployment of ground troops ahead of any settlement with President Milosevic.

In Washington, Mr Blair, who has been among the strongest advocates of Nato's current approach of air strikes, is expected to argue the deployment of ground troops would not be a full-scale invasion.

He will say that Serb forces have been so reduced in strength by the continued bombing they would be unable to prevent a ground force entering Kosovo.

Speaking ahead of his trip to Washington, Mr Blair justified Nato's actions on Russian television.

President Boris Yeltsin has previously warned of the danger of the Balkan conflict escalating into a world war and would be likely to opposed a ground force.