JOHN MAJOR returns from the Maastricht negotiations to face another tough battle, this time against British anti-European-Community Conservatives led by his predecessor, Margaret Thatcher.Even before his return, the opposition Labour Party began assailing Mr. Major for allowing pressures from Mrs. Thatcher and her supporters to push Britain into a diplomatic corner. Labour's John Smith, shadow chancellor of the exchequer, said: "Britain was isolated at Maastricht because Thatcherism appears to be alive and well and living at 10 Downing Street." Criticism from the Iron Lady of British politics promises to be harder for Major to handle than Labour Party attacks, and is likely to deepen an already serious split within the Conservative Party on European issues. Sir Norman Fowler, chairman of the Conservative Party's European Affairs Committee, welcomed the outcome of the talks on a single currency, but Thatcher's friends sang a different tune. One Thatcherite Conservative member of parliament (MP) said on Dec. 10 that the ex-premier intended to challenge Major on the single-currency plan endorsed by the finance ministers of the 12 nations at Maastricht, Netherlands, on Dec. 9, and expected to be approved after lengthy negotiations Dec 10. The MP said Thatcher would lead a House of Commons rebellion on the issue if his answers did not satisfy her. The revolt, the MP said, could include a Commons vote by an anti-EC Conservative group against the Major government. Thatcher would not be satisfied with an opt-out clause enabling Britain to stay outside a currency union, the MP said, adding: "She will demand a referendum on a single currency and will not be satisfied with assurances that Britain need not abandon the pound sterling until 1999." Asked whether Thatcher was worried about the damage her challenge would have on the Conservative government's reelection prospects next year, the MP said: "She [Mrs. Thatcher] has considered that, but believes a vital point of principle is involved." Although the government, which normally enjoys a 100-seat margin in the Commons, would probably survive a Thatcher-led rebellion, the ruling party would be damaged by proof of the profound divisions in its ranks. Thatcherite MPs made no public criticisms of Major while he was at Maastricht, but there was plenty of privately expressed dismay at the cracking pace on a single currency set by the other leaders. The dismay was shared by senior government figures who said the single currency timetable proposed by the other leaders was not realistic. David Howell, Conservative chairman of the House of Commons Foreign Affairs Committee, said: "I doubt whether the targets can be met." He said however that he would support the prime minister in a Commons vote. Among the group of about 40 Conservatives who might join Thatcher in a challenge to Major there was resentment at reports that the former premier has been excluded from a prominent role in next year's general election campaign. The exclusion has prompted talk of a cold war between Major and Thatcher. Chris Patten, the party chairman and a staunch Major supporter, made the decision to sideline Thatcher in the coming campaign. Conservative sources said this was because two weeks ago in the House of Commons she had embarrassed Major by suggesting that he "use a handbag" to defend Britain's interests at Maastricht. The demotion of Thatcher during the campaign is certain to dismay many loyal Conservatives. The estrangement between Major and Thatcher is becoming personal as well. The prime minister and his wife, Norma, were not invited to a 40th wedding anniversary party for the Thatchers on Dec. 12, although other leading ministers and their wives were. Labour is relishing Major's difficulties in reining in his outspoken predecessor. Piet Dankert, Dutch minister for European affairs, said Dec. 9 on British television that he sometimes thought Thatcher was "still there." Her opinions on a single currency were limiting Major's "scope for maneuver," he said. Mr. Dankert's remarks were cited by Labour as evidence that Major was under heavy pressure from Thatcher to renounce policies he supported but she did not like.