Mr Ashdown was speaking amid final efforts to bring a peaceful end to the crisis in Kosovo.

Concern is growing that the latest Serbian offensive aims to destroy the separatist Kosovo Liberation Army before Nato can attack Serbia.

The United States special envoy, Richard Holbrooke, is flying to Belgrade on Monday, where he will deliver a final warning to Yugoslav President Slobodan Milosevic.

At least 20,000 ethnic Albanians are believed to have fled their homes in north-western Kosovo.

Paddy Ashdown: "Morally and practically impossible for the Serbs to continue to rule Kosovo"

Mr Ashdown said: "The action of the Serbs over the last two years in Kosovo has made it both morally and practically impossible for the Serbs to continue to rule Kosovo on the basis of 5 % of the population."

Mr Ashdown said air strikes could not be used in a vacuum and must have a further political aim.

Paddy Ashdown: Visited Kosovo three times

Air strikes could only persuade President Milosevic to return to the negotiating table by "requiring him to pay such a high price in military assets", he told BBC Radio 4's Today programme.

Mr Ashdown said he believed the chances of that were very slim but it was a "perfectly fair military aim".

The next step was to establish an international protectorate in Kosovo through the use of ground troops to expel Serbs.

That could not be done with air strikes alone, said Mr Ashdown.

Without following up air strikes with ground troops, Nato would become an air force for the Kosovo Liberation Army, he said.

'Final solution'

"Unless you do [send in ground troops] then what you will see on your television screens is President Milosevic using weapons of total war to achieve a final solution on ethnic grounds in Kosovo ... then you'll see a widening Balkan war."

Mr Ashdown has visited Kosovo several times, meeting with President Milosevic on one occasion.

He told Today: "It is my view that if we get this wrong, we are now on the threshold of a major ground conflict, or at least a major conflict, on the mainland of Europe.

"We're closer to that than we have been at any time in the second half of this century."

The Serbian army was the probably the fourth most powerful army in the former Warsaw Pact and the consequences of casualties had to be faced, he said.