Prime Minister Margaret Thatcher is proposing a big-power conference with the two Germanys to work out a framework for German reunification, government sources said today. She wants the United States, Soviet Union, Britain and France _ the original occupiers of Germany after World War II _ to meet at foreign-minister level with representatives of East and West Germany, said the sources at her Downing Street headquarters. They spoke on condition they not be identified. Foreign Secretary Douglas Hurd drew up a detailed proposal and discussed it with Britain's allies at this week's East-West ``Open Skies'' conference in Canada of 23 nations, the sources said. The proposal is expected to be amplified when Mrs. Thatcher meets Foreign Minister Hans-Dietrich Genscher of West Germany in London on Wednesday. Genscher asked for the meeting, the sources said. They said no date was being set for the conference, but it was hoped it would take place soon after East Germany's March 18 elections. Any formulas agreed upon would then be presented to the 35-nation Conference on Security and Cooperation in Europe. Mrs. Thatcher's proposal is seen as a response to critics who accuse her of seeking to slow the momentum toward reunification. The British leader has not disguised her concern that hasty moves could undermine European security. On Saturday, at a meeting of her Conservative Party, Mrs. Thatcher stressed that the 1975 Helsinki agreement stipulated that ``no boundaries would be changed except by agreement.'' ``So if any boundaries are to be changed, this requires massive consultation between us,'' she said. ``All this means the changes they are having now in Germany and moves towards unification must be done in accord with the other obligations to which we signed up.'' The Daily Mail, which first carried the report today, said the conference could produce an agreement ``tantamount to a German peace treaty.'' No peace treaty was signed after World War II, and although the United States, Britain and France renounced their status as occupiers in 1955, they retain authority in Berlin and maintain armies in West Germany. The Daily Mail said Mrs. Thatcher's proposal was a way of saying: ``Let's be practical and settle the complex problems at the conference table rather than through demonstrations in the streets.''