There is no question of Britain using its UN Security Council veto against a resolution to enforce the no-fly zone over Bosnia - shooting down Serbian aircraft that have flouted the UN zone since its imposition in October.
But, say Westminster and Whitehall sources, the Prime Minister wants to be sure if the Serbs are tempted to retaliate against vulnerable UN forces or aid workers, they will be left in no doubt of the UN-sanctioned retribution.
Mr Major also wants to bolster those military threats with political action to close the noose on Belgrade. Government sources said that ministers wanted the Serbs to be turned into an 'international pariah' - by sealing frontiers, a shut-down of all international communications links, and a closure of all embassies.
Nato sources have confirmed the alliance has begun informal planning in three areas - use of aircraft to enforce the UN ban on military flights over Bosnia, creating civilian 'safe havens', and preventing the war from spreading to Macedonia and Kosovo.
The shift in emphasis in London was reflected in the international arena, when Britain yesterday yielded for the first time to pressure from allies for the resolution to enforce the no-fly zone.
But Douglas Hurd, conscious of objections among defence - as distinct from Foreign Office - officials, made his reservations known in talks with his US and French counterparts, Lawrence Eagleburger and Roland Dumas, at the Conference on Security and Co-operation in Europe meeting in Stockholm yesterday.
British officials hinted that a UN resolution enforcing the no- fly zone will probably emerge by next week, after further consultations in Geneva and Brussels.
Mr Hurd told a press conference that while the Security Council 'ought to examine the results of the no-fly zone', one important aspect of enforcing it was the effect on humanitarian effort.
The difference between Britain and its allies reflects the dilemma over what benefits the no-fly zone would bring, sending a signal to the Serbs that they cannot flout UN authority, against the risk of retaliation to troops on the ground.
The Ministry of Defence fears the risks to peacekeeping troops may outweigh the benefits of enforcement. A British source said Serbian violations posed 'more of a political than a military problem'. He added: 'Our military advisers do not tell us that these flights are having any significant effect on the fighting. The examination is under way - that's what the French are up to.' Britain was guided by consideration of the humanitarian effort and by its having 'more troops on the ground in Bosnia than anyone else'.
Mr Major and the MoD are very reluctant to step up military intervention, but it is understood that a meeting of the Cabinet's Overseas and Defence Policy committee yesterday recognised that such action was inevitable.
After the increase in pressure for firm action, John Smith, the Labour leader, yesterday cited growing domestic and diplomatic 'feeling' on breaches of the no-fly zone to support his view that 'the time has now come for effective international action'.
Mr Major told him: 'We have never ruled out the possible need to enforce the zone. But we, with our allies and partners, do have to consider very carefully how it is to be enforced and what the effect of that enforcement would be, both on the humanitarian effort, none of us wish to see that end either this side of Christmas or long after, and on the safety of our troops.'
He then told the Commons of 'other measures' being considered; a reference to the proposed 'stranglehold' on Belgrade.
Public pressure was also cited as a reason for action by Tom King, a former Conservative Secretary of State for Defence - in defiance of 'every military rule of what is sensible or possible'.
Paddy Ashdown maintained the heat on the Government, broadcasting attacks on the Serbs from Sarajevo, saying: 'They sit with impunity on the hillsides knocking down a city for their sport.'
The Liberal Democrat leader added: 'The international community ought to say, 'Sarajevo and the other besieged cities are under (UN) mandate, and after a given time, say midday in 48 hours, all the heavy weapons will be withdrawn, and those that aren't will be subject to attack from the air'.'
For the moment, Mr Major is keen to pursue a cautious, step- by-step approach - going along with US and French demands for firmer action, but with back-up protection for troops on the ground.
One Whitehall source said last night that contingency plans had to be drawn up for the possibility of Serbian retaliation.
Although it was accepted the Serbs were not as strong as portrayed, they could inflict a lot of damage in a short time. 'We have to make sure that if they do counter-attack, they are given a very quick and very bloody nose,' the source added.
That was a view shared by Mr Ashdown, who told BBC radio: 'Agreements on paper mean nothing; agreements spoken mean nothing. Only if you take the power to enforce those agreements will peace come . . . . And sooner or later, we're going to have to do it.
'We're approaching the crucial final moments of a great European city. The question for us is: Are we prepared to stand by and see it happen?'
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