WASHINGTON (AP) _ President Reagan has sided with environmentalists in ratifying U.S. efforts to seek deep international cuts in emissions of ozone-destroying chemicals, government sources said Wednesday.

Officially, the president's instructions to Richard Benedick, deputy assistant secretary of state for oceans and environmental and scientific affairs, remain classified to avoid giving away the U.S. negotiating position.

Benedick was in Brussels, winding up two days of meetings with key members of other delegations participating in talks sponsored by the United Nations Environment Program.

Sources in a position to know, who insisted on anonymity, said Reagan had backed the course that the State Department and Environmental Protection Agency already were following.

The issue probably would not have come to Reagan's attention but for Interior Secretary Donald P. Hodel, who insisted on consideration by the Domestic Policy Council and presentation of alternatives to the president.

High-altitude ozone shields the earth from harmful ultra-violet rays from the sun. Long-lived chemicals of the chlorofluorocarbon class destroy the ozone, letting increased ultra-violet through to cause more skin cancers around the world.

Hodel had been reported as backing a policy promoting increased use of suntan lotions, hats and sunglasses as an alternative to emission reductions. He insisted he was misunderstood and was only trying to present a fallback position in case no international agreement could be reached.

Hodel's spokesman, David Prosperi, declined to comment beyond a one- sentence statement from the secretary: ''I am extremely pleased that after he had an opportunity to consider the issues concerning ozone-depleting chemicals, the president issued clear instructions to the U.S. negotiators directing them to seek an effective international agreement.''

''We got all we asked for,'' said one source opposed to Hodel.

The international talks already have tentatively agreed on a 20 percent cut in chlorofluorocarbon emissions and were scheduled to consider a further 30 percent reduction.

The second-stage cut could come as the result of a positive vote by a majority of participating nations, or from a ''negative trigger'' - that is, automatically unless vetoed by two-thirds. The president chose to seek a ''negative trigger'' favored by environmentalists.

Details of the Brussels talks were not immediately known. Reporting cables had not arrived. One source said the Brussels negotiators had reached a ''fragile consensus'' in line with the U.S. position, but did not know if the ''trigger'' question had been settled.

They did agree, this source said, that trade in bulk chlorofluorocarbon chemicals and products using them from nations not abiding by the expected international agreement would be restricted by nations taking part in it.

A further session at The Hague is planned next week for legal experts to decide how many nations it will take to put an agreement into effect, what voting arrangements should be used in overseeing it and other important but subsidiary questions.

Still other negotiating sessions will be necessary in Montreal in September before the planned wrapup signing session there.