Common Market nations tentatively approved a new world trade agreement today, clearing the way for formal signing of a multilateral treaty with the United States and Japan next week.

Roy Jenkins, president of the European Economic Community, called the treaty talks "the biggest and most ambitious of post-war trade negotiations."

Diplomatic sources here and in the United States said the formal signing has been scheduled for April 11 in Geneva.

But European presence in Geneva next week could depend on the lifting of a reserve placed by the Italian government on the MNTN package agreed on by its eight European partners in Luxembourg last night. However, Italian objections, which concern trade in grapes and woolen textiles, are not expected to jeopardize the overall negotiating achievement.

"We hope that the Italian government will feel able to lift its reserve," said Jenkins. EEC trade officials privately buttress this hope with predictions that the Italians will fall into line by the end of this week or early next at the latest.

Jenkins said the package on which the EEC now seems agreed includes a "major tariff reduction," a "substantial agricultural settlement," and a "major strengthening of the GATT" (the General Agreement on Tariffs and Trade), the body which governs world trade. Reinforcement of ordered international trade would be assisted by the establishment of codes on standards, customs valuation, countervailing duties and government procurement, he added.

The new trade agreement would be signed formally only after a ratification process expected to be completed by next fall. The pact would give the world "a greater chance of a freer trading system in the 1980s," Jenkins said. Equally important is avoiding protectionism and ensuing economic recession, he warned.

Under the new arrangement, the ECC's import tariffs against U.S. goods are expected to be cut "by about one-third," said top Common Market trade negotiator Wilhelm Haferkamp. He described the U.S. as "our most important overseas sales market." But duties against imports from Japan-which has exacerbated protectionist sentiments in Europe by a concentrated export sales strategy-will fall only "by about a quarter," Haferkamp surmised.

Overall the average EEC tariff for industrial goods is expected to drop from 9.8 percent to 7.5 percent, the EEC negotiator said.

On trade in agriculture, the new package would mean an end to the "trench warfare" which has characterized trade relation particularly between the U.S. and EEC in recent years, claimed the Common Market's agricutural chief, Finn Gunndelach, who was also present at today's major conference.

He said the U.S. and the EEC have achieved balanced concessions, and the stage is set for "the beginning of the rule of law for trade in agriculture." CAPTION: Graph, The Market In Brief April 4, The Washington Post