U.S. negotiators at an international conference to curb ozone- depleting chemicals yesterday proposed to increase substantially the number of countries that would have to ratify a treaty for it to take effect.

The proposal, which would require treaty ratification by nations representing 90 percent of the world's production of chlorofluorocarbons (CFCs), is designed to assure "effective international cooperation" and fair treatment for the manufacturers of participating countries, said Dave Cohen, a spokesman for the Environmental Protection Agency.

But environmentalists immediately criticized the proposal as a tactic to delay or scuttle international controls on CFCs, which destroy the Earth's stratospheric ozone, which screens out most of the sun's harmful ultraviolet rays.

"This is the next best idea for derailing the ozone agreement since the hats and sunglasses proposal," said David Doniger, of the Natural Resources Defense Council, referring to a suggestion within the Reagan administration to adopt cosmetic defenses to ozone depletion in place of strong pollution controls.

Yesterday's proposal came at a planning session in Montreal for next week's Diplomatic Conference on Stratospheric Ozone. Diplomats from 31 nations will meet in the hope of crafting a final treaty.

A draft agreement that has been circulating since June would make the agreement effective one year after its ratification by nations representing 60 percent of world output of CFCs, which U.S. negotiators now want to raise to 90 percent. The draft calls for an immediate CFC production freeze at 1986 levels and a 50 percent cutback of CFCs within a decade.

U.S. companies produce a third of the world's CFCs, which are used as cooling agents in refrigerators and air conditioners and propellants in aerosol cans. Common Market countries represent 42 percent of world production, Japan 11 percent and the Soviet Union 10 percent.

Cohen said the 90 percent proposal would assure that U.S manufacturers of CFC would be asked to do no more than foreign competitors. This would make an agreement more palatable to the Senate for ratification, he said.

"We want a fair agreement with participation by all major parties," he said.

Doniger said the proposal could greatly delay the effective date of a treaty. Neither the Soviet Union nor Japan has endorsed the draft agreement. Moreover, he said, the proposal places power in the hands of other nations to trigger a vitally necessary environmental measure.