The United States has won support from a majority of members of the General Agreement on Tariffs and Trade (GATT) for a formal meeting next month to prepare a new round of world trade talks, according to GATT sources.

Forty-six of the 90 GATT members have voted in favor of the meeting, which is tentatively scheduled for Sept. 30, the sources said. More acceptances are expected later.

The United States had called for the ballot at a meeting of the governing council of GATT on July 19. Decisions in the body are traditionally taken by consensus, but when Third World nations blocked a U.S. move to convene the preparatory meeting, U.S. Ambassador Peter Murphy called for a vote. He told reporters at the time that Washington was confident it had enough support for the meeting despite objections, principally from Brazil and India.

Washington will attempt at the September meeting to win acceptance for inclusion of services -- including finance, tourism and shipping -- in the new GATT round. The Third World opposes this plan, and a tentative agreement to have two parallel sets of talks -- one on trade, one on services -- unraveled in July when, GATT sources said, U.S. Trade Representative Clayton Yeutter overruled his subordinates on the parallel talks.

The September meeting, which will be at "senior official" level -- the United States is expected to be represented by Deputy Trade Representative Michael B. Smith -- will seek agreement on the formal start of the new round of tariff cutting talks early in 1986. This agreement would then be ratified by GATT member states at their regular year-end meeting in November. Given that Washington already has forced a vote on holding the meeting -- and won -- it presumably can succeed on the services question in September, GATT sources said.

The European Community, Canada, Australia, the Nordic countries, Spain and Portugal all back the U.S. position that services be included. Brazil says that GATT should concern itself exclusively with trade.