FOR THE FIRST time in her phenomenally long tenure as prime minister of Britain, Margaret Thatcher has lost control of a crucial element of her Conservative party. It's come about in a curious and paradoxical way.

She rose to power more than 11 years ago as the voice of the practical wing of the party, mostly business people pushing for faster economic growth and insisting on the discipline of the market. They were fed up with the sentimentality and nostalgia of the high Tory tradition. But those are the same people who see the European Common Market as the salvation of the British economy and have been pressing for closer integration. Mrs. Thatcher is, among other things, a stubborn nationalist. As France and Germany have increasingly warmed to the idea of political and economic union of the Common Market, Mrs. Thatcher has increasingly resisted it. The result has been to drive a wedge between her and the people who are her natural supporters.

The differences between them have become unprecedentedly sharp and explicit over the past month. In late October the Common Market's other 11 members, over Mrs. Thatcher's furious objections, voted to proceed with the next step toward union. It involves establishing a central bank for the 12 countries and would make possible a common currency before the end of the decade -- an idea that Mrs. Thatcher denounced as an intolerable sacrifice of sovereignty.

In response, her most experienced lieutenant, Geoffrey Howe, resigned with a letter arguing that her strident opposition will isolate Britain and diminish its influence over the great changes that Europe is now undertaking. There have been plenty of resignations over the years from Mrs. Thatcher's cabinets, but this one is different. It puts her at odds with the people who are the modernizers and reformers in the party, and has set off an explosion of dissent. It's possible that she will face an open challenge for the leadership of the party at its annual caucus next month. Even if she manages to forestall that kind of a fight, she has clearly suffered a loss of strength from which it will be difficult to recover.

She has been in trouble many times before, but she was always able to maintain tight control of the party because she could win elections for it. She has won three in a row, a record unmatched in modern British politics. But there will be another election sometime in the next 19 months, and since Europe will certainly be an issue, many of the most attractive and foresighted elements of her own party are likely to be, reluctantly, against her.