LONDON — THE bitterness of the fighting in Yugoslavia has convinced Lord Carrington, chairman of the European Community peace conference, that only a last-minute willingness by Serbian and Croatian forces to return to barracks can save the country from full-scale civil war.It also appears to rule out, for the moment at least, any chance of the EC being able to send in a peace force to contain the fighting. Carrington gave a gloomy assessment of future peace prospects after returning to London from Yugoslavia, where he had brokered another attempted cease-fire. The former British Foreign Secretary spoke to the Monitor prior to flying to The Hague yesterday for a reconvening of peace talks between Yugoslav and EC foreign ministers. "How can we go on having a peace conference if everyone is killing each other?" asked Carrington, also a former NATO secretary general. "I don't think the Serbs and the Croats have negotiation on their minds. If the cease-fire collapses totally, we shall probably see the bloodiest civil war in Europe for a very long time. It is a terrible prospect." On Tuesday, Carrington obtained the signatures of Gen. Veljko Kadijevic, the Yugoslav federal defense minister, Croatia's President Franjo Tudjman, and Serbian President Slobodan Milosevic on a cease-fire agreement designed to take force at noon local time on Wednesday. But as the deadline approached, fighting intensified. Yugoslav air units and ships bombarded Croatian coastal towns. The Croatian capital, Zagreb, was attacked by pro-Serbian jets. Most of the fighting was in eastern and southern Croatia and along the Adriatic coast, where the port of Split was a prime target for pro-Serbian warships. Carrington said Wednesday it was too early to conclude that the cease-fire was null. It should be given 24 hours to take effect, he said. But as Carrington prepared to head for The Hague he struck a gloomy note about what was likely to happen if the cease-fire collapsed entirely. Outside intervention by an EC peace force would not help at the present stage, he said. "I do not believe armed intervention by the EC or anyone else would be effective if the fighting carries on at the current level of intensity," he added. CARRINGTON was backed up by British Prime Minister John Major, who said he was against a Dutch government proposal to send a lightly-armed EC peace force to Yugoslavia. The British government's negative stand appeared to rule out any prospect of EC intervention for the time being. On Monday, a proposal to send troops was made by the Netherlands, which currently holds the EC presidency. Britain's defense minister, Tom King, speaking during a tour of the Far East, drew a distinction between peace-keeping, which could occur once a cease-fire was effective, and peacemaking, intended to halt fighting while it was still continuing. "Before you can keep the peace there must be a cease-fire. When intense fighting is going on there is a risk of the external forces being drawn into the conflict," he said. "It is a dangerous time. As long as the fighting goes on there's a risk that it will somehow catch fire elsewhere." Willem van Eekelen, secretary general of the Western European Union, a defense group, said he saw "several problems for any plan to send a European peace force to Yugoslavia." Unofficial British estimates of what would be required to mount an intervention in Yugoslavia speak of a force of at least 50,000 men, with full equipment and logistical backup. Edward Cowan, a defense analyst and former British military attache in Yugoslavia, said such a force "could expect to sustain heavy casualties" and it would be very likely to have to remain in Yugoslavia "for a very long time." "In my opinion, the British public would not support such a commitment," Mr. Cowan said.