On the eve of today's emergency Commons debate, called after the capture of 33 British hostages on Sunday, Mr Major told a rare Downing Street news conference that he had sent a letter to the Bosnian Serb President, Radovan Karadzic, and his army commander, General Ratko Mladic. He said the letter held the two men "personally responsible" for the safety of the hostages.
The Prime Minister said the seizure of the UN troops was an "outrage" and a "self-defeating folly ... the path to total international isolation and permanent pariah status."
Nato foreign ministers, meeting in Noordwijk, in the Netherlands, took an equally robust line. Officials said that one possibility under discussion was the use of the strengthened UN force to punch a corridor through a short stretch of Serb-held territory to lift the siege of Sarajevo.
No firm decisions were taken but the UN mission in Bosnia is likely to be reshaped to make it easier to defend. Officials insisted this did not mean the abandonment of outlying "safe areas" such as Gorazde.
Rumours that the US was considering a commando operation to rescue hostages were not completely ruled out by American officials, but this seemed a far-fetched scenario. More usefully, the US has offered high-tech equipment, helicopters and armoured cars to bolster the UN troops in Bosnia. Britain and France are also examining the idea of a rapid reaction force, perhaps with US support.
Nato officials said the prospect of total UN withdrawal, though not abandoned, is being held in abeyance. "The accent is on concentrating the UN, reducing its vulnerability and reinforcing it," said Douglas Hurd, the Foreign Secretary.
Despite this show of Western resolve, there was no sign of any slackening in the defiance of the Bosnian Serbs. Captured UN soldiers, used as human shields, were still being held in chains, despite a pledge to free them. The Bosnian Serb leadership repudiated all its working agreements with the UN and threatened to fire upon Nato aircraft flying over its territory.
In another extraordinary development, the self-proclaimed Bosnian Serb "government" sent what amounted to a death threat by fax to a UN spokesman in Sarajevo, who has spoken out bluntly about their activities in recent days.
The fax to Alexander Ivanko, a Russian, signed by the information minister in Pale, Mirosolav Toholj, said: "You should better go home before it is too late. If Mr Ivanko keeps on behaving like this, he might run into an accident."
Mr Major's promise to keep British troops in Bosnia "for as long as they can remain there without unacceptable risks" came despite mounting rumblings within his own party. The Prime Minister said British troops were there to guard a "vital national interest".
"I believe we have both a strategic interest in preventing a wider Balkan War and I think we have a humanitarian obligation."
Mr Major said the five-nation contact group was now closer than "at any stage in the past" to persuading the Serbian President, Slobodan Milosevic, to recognise Bosnia.
The Prime Minister is guaranteed Labour's support today. But he will have to convince his backbenchers against the background of a right-wing effort to make support for the reinforcements conditional on their being used to cover a withdrawal.Reuse content