General Sir Jeremy Mackenzie, Nato's Deputy Supreme Allied Commander, Europe until last year, tells the programme that the allliance planned for a very large force if an invasion of Kosovo was ordered.

Asked about the extent of the force, he said: "Very extensive because it wasn't actually a plan to merely invade Kosovo, although that was one of the options.

"The heaviest option was of course the entire invasion of Serbia and with the ... armed forces they had, this was a serious undertaking.

"Of course, it is an extraordinarily difficult place to get to, so the option was the heavy end - 180,000 to 200,000 (troops) was the figure you heard bandied around."

A Ministry of Defence spokesman confirmed the plans had been examined: "Last year we looked at all the options as you would expect, including a ground force invasion of up to 200,000.

"Following this analysis we concluded that the air campaign was the best option," he said.

Error of judgement

Panorama also heard criticism of the current Nato campaign and some of the key figures in the alliance.

US Secretary of State Madeleine Albright is described by a Congressional leader as making an error of judgement in her assesment of the character of Yugoslav President Slobodan Milosevic.

The Chair of the House select Committee on Intelligence, Porter Goss said: "I think she firmly did believe there was a possibility that he would cave in, with, maybe if not bombing light, then bombing moderate, shall we say?

"But that didn't come to pass and the question has always been, 'What if the bombing doesn't work? What if Milosevic doesn't roll back his forces? What do we do then?'

"The answer has always been a little blurred," he said.

The Panorama programme "War Room" is due to be broadcast at 21:05 GMT (22:05 BST) on Monday, 19 April.