His warning comes after the most intensive strikes yet by RAF Harriers and other Nato planes against Serb forces.

Mr Robertson plans to use a speech at Harvard University to stress an air offensive can force Serb President Slobodan Milosevic to accept Nato's demands.

But contingency plans should exist in case ground troops are judged the only means of forcing an end to the conflict.

George Robertson: Speech at Harvard University

His comments form part of a gradual shift among Nato leaders, who initially ruled out sending in forces unless President Milosevic had accepted defeat.

But a Ministry of Defence spokesman insisted no plans existed presently for an invasion.

At a briefing on Friday, Foreign Office minister Tony Lloyd described how Nato had stepped up its air attacks on the night after it admitted one of its pilots had mistakenly dropped a bomb on a convoy of refugees.

"Nato is still intensifying the campaign. The pressure stays on and it gets stronger," he said.

Mr Lloyd expressed regret for civilian deaths, but blamed Serb forces for creating the situation.

"It is worth recalling that while Nato makes every effort to avoid civilian casualties, Milosevic deliberately targets the civilian population of Kosovo," he said.

Harriers had their busiest night since the conflict began

Chief of the Defence Staff General Sir Charles Guthrie detailed British involvement in the strikes.

RAF Harriers flew 18 sorties, successfully attacking a number of targets, such as artillery and anti-aircraft sites and command facilities in Pristina, he said.

"We continue to play a significant role in the campaign. This campaign will continue until Milosevic agrees to put an end to the misery and agrees to the demands of the international community."

All Nato and British aircraft that took part in overnight strikes returned safely, Sir Charles said.

He went on to pay tribute to the efforts of British forces providing emergency assistance to refugees who have fled from Kosovo to the Former Yugoslav Republic of Macedonia.

More than 129,000 meals had been provided by UK soliders, who had dug 130 deep latrines and eight water supply facilities, he said.

"Quoting statistics like this can only give you the very merest glimpse of the amount of work our forces have done to help the refugees.

"I know their contribution has been crucial and is indeed very much appreciated."

Mr Lloyd, who had just returned from a visit to the refugee camps, stressed the individual horror behind the war.

He said there were reports that thousands of young men had been massacred, including whole villages.

Those who escaped, often women and children, he described as "quiet heroes".

"We must never and we as a government will not lose sight of the individual stories when we talk about the refugees.

"The numbers are overwhelming and they blind us to the reality. But these are our fellow Europeans, some three hours away by air.