WHEN John Major gets to Maastricht he will be a lonely leader with a steep hill to climb.That was the bleak message from British and French diplomatic sources after Britain's prime minister spent two and a half hours with President Francois Mitterrand Dec. 2, fruitlessly trying to enlist French help in averting his isolation at the European Community summit beginning Dec. 9 in the Netherlands. The Maastricht summit is intended to produce binding EC agreements on economic and political union. But the gloom generated by the Major-Mitterrand meeting appeared to decrease the likelihood that Britain can achieve a deal which its prime minister can later sell to British legislators and confidently place before the voters at next year's general election. British parliamentarians, anxiously monitoring the hectic diplomatic exchanges in the run-up to Maastricht, said Mr. Major appeared to have made hardly any headway with Mr. Mitterrand. They noted also that at other diplomatic encounters, British ministers have found themselves seriously at odds with their European counterparts. Major's officials indicated that unless breakthroughs came on the eve of Maastricht, as many as half a dozen divisive issues would have to be resolved by the 12 EC heads of government during their meetings Dec. 9 and 10. In his talks with Mitterrand, Major argued that France could help Britain by persuading Germany's Chancellor Helmut Kohl in particular to accept a slower pace for European economic and political integration. A French diplomat close to the talks in London said afterward that Mitterrand wanted to help Britain but felt that he had obligations to Mr. Kohl. The diplomat expressed concern that Major will find himself alone on a widening range of key questions at Maastricht. That belief also appeared to be strengthening among key government members of the House of Commons. Although reluctant to make public comments while Major and his ministers were in the midst of crucial negotiations, some MP's privately expressed alarm that the British team at Maastricht would either have to accept the majority EC view on key questions or refuse to sign the proposed union treaties. A member of the House of Commons foreign affairs committee who favors closer European integration said: "There is a real danger of an anti-EC backlash in Britain if the prime minister returns from Maastricht empty-handed." After Major's encounter with Mitterrand, the two leaders ordered a blackout on detailed official comment. A British spokesman said, "They made some progress, but agreed that further work must be done. They will keep in touch in the run up to Maastricht." A French spokesman was equally uncommunicative. Even before Mitterrand reached London it was apparent that Major and his senior ministers were making slow progress in their attempts to win allies interested in watering down EC proposals aimed at propelling the Community toward closer union. EC finance ministers Dec. 1 rejected by 11 votes to 1 a British proposal to create a general escape clause from agreements on a single Community currency. They told British Chancellor of the Exchequer Norman Lamont that if Britain wanted to opt out, it would have to accept a clause that pinpointed its own isolation. Sources monitoring the Mitterrand-Major exchanges said the leaders disagreedon EC foreign, defense, and social policy. Major had argued that any EC country should be allowed to opt out of an agreed Community foreign policy stance if it felt its vital national interests were threatened. Mitterrand is thought to stand closer to Kohl, who wants EC foreign policy to be determined by majority voting. The British leader also insisted that the North Atlantic Treaty Organization should remain the cornerstone of European security policy. Mitterrand argued for a greater defense role for the EC. Major then went on to criticize EC proposals that all 12 Community members should accept a social charter laying down standards for workers, including a maximum 48-hour work-week. The prime minister's officials say this is a politically sensitive issue for his government, which is deeply opposed to what it sees as Brussels's interference in domestic policy. Mitterrand, sources said, sees the social charter proposal as progressive and reminded Major that Kohl supports it.