In an address to the nation, the prime minister appealed to the whole country to back the action, saying "barbarity cannot be allowed to defeat justice".

"It's simply the right thing to do," he said.

To fail to intervene would, he said, "show unpardonable weakness and dereliction - that is not the tradition of Britain".

Tony Blair: "We must act for the sake of humanity"

Mr Blair began by laying the blame for the Nato bombing raids against Yugoslav forces squarely at the door of Serbian President Slobodan Milosevic.

He dismissed arguments that it was an illegal attack on a sovereign nation, saying that Mr Milosevic had "scrapped Kosovo's autonomy" when "90% of the people are not Serbs".

He said Nato had been forced to step in to protect the Kosovo Albanians - "our fellow human beings" - who "now have no rights, no justice, no protection".

'Spiralling' conflict

He quoted figures of 250,000 homeless and 2,000 killed "since last spring".

Tony Blair: "This is simply the right thing to do"

"If we do nothing, Milosevic will feel free to do what he likes with the civilian population. They will be ground under his heel," he said.

And Mr Blair raised the spectre of a "spiralling" conflict with "hundreds of thousands of refugees" that would not stop at the borders of Kosovo if Nato failed to stop him "killing innocent people and driving them from their homes".

He pointed out that World War I had started in the Balkans, in Sarajevo, and that "we only just averted war again" during the Bosnian conflict five years ago.

'Kosovo is part of Europe'

The prime minister's address was a point-by-point rehearsal of the arguments used by Nato leaders since the decision was taken to use force to make Mr Milosevic stop the Serb forces' brutal "counter-insurgency" operation in Kosovo.

But in a subtle departure he spoke of Kosovo as "part of Europe ... a short sea journey from Italy. A short drive from Greece."

"We must act for the sake of humanity and the future safety of our region and the world," he said.