The General Agreement on Tariffs and Trade (GATT) agreed today to convene a preparatory committee in January to chart the course of a new round of world trade talks scheduled to begin in the fall of 1987.

The agreement left unanswered the thorny question of whether the booming world trade in services -- such as insurance, banking and tourism -- would be included in the new round. But Deputy U.S. Trade Representative Michael B. Smith, head of the American delegation, told The Washington Post that he had no doubt the subject would be discussed.

"We won, and it's as simple as that," he said. "We have a commitment to discuss everything . . . and that includes services." Third World opponents of the U.S. view were equally adamant however that they had won.

"We have succeeded in having a preparatory committee formed without any preconditions, and that is what we set out to do," Brazilian Ambassador Paulo Nogueria Batista told reporters. "There is no change in our position, which is that GATT is about trade in goods, not services." Brazil, with the support of India, Yugoslavia, Argentina and Egypt has been blocking agreement on the inclusion of services in GATT for more than a year.

The United States has warned that unless services are included in the next GATT round, the country won't participate, but will instead seek nation-by-nation trade pacts.

When the annual general meeting of the 90-nation organization began Monday, the five were still claiming that they were not prepared to let the preparatory committee address the matter.

But Smith said he had the support of the necessary 51 percent of the member countries to force inclusion of services if necessary, and added that he was prepared to call for such a vote. Gatt traditionally operates on consensus, and the organization's chairman, Felipe Jaramillo of Colombia, promptly launched a series of behind-the-scenes negotiations to head off a vote and find a compromise.

He succeeded, after the five countries said they would agree to a vaguely worded final communique that said the preparatory committee would discuss "the subject matter, modalities for, and participation in the multilateral trade negotiations." Both sides found this satisfactory.

Smith said the language meant the preparatory committee, expected to convene here Jan. 15 under the chairmanship of GATT Director General Arthur Dunkel, could discuss services or anything else. Batista said he took it to mean that nothing has been decided and that any question of discussing services in GATT was put off to next year.

"It's a question of nuance," a senior GATT official, who asked not to be named, said. "The five were reluctant to see things go to a vote because they knew that not only would they lose, but GATT's credibility would be threatened. The Americans were happy to see a vague wording they could go with which would allow them to claim victory in Washington and, at the same time, return to fight the services battle again in January."

The preparatory committee, which will probably be made up of officials of the embassies of the 90 countries in Geneva, was mandated to prepare an outline of what a new round of trade talks would be concerned with by no later than mid-July 1986. This would be presented to a ministerial meeting of the member countries -- who will number 91 by then since Mexico has applied for membership -- next September. The new round would then start some time in the fall.