The Reagan administration's proposed cuts in the space budget have triggered a difference of opinion between the United States and its Western European allies over how economies should be made in the $6 billion U.S. space program.

So serious is the split that the European Space Agency has formally protested the proposed cuts to the State Department, and France has threatened to cease cooperation with the United States in international space missions. Last week, the ambassadors from Sweden, Switzerland and Italy delivered and aide-memoire to the State Department urging that budget cuts affecting such missions be restored.

Nowhere would the proposals cut so deeply into U.S.-European space relations as in the administration's decision not to finance a cooperative mission to study Halley's comet, which swings by the sun on its closest approach to earth in March 1986.

Critics say the decision means that the most serious study of Halley's comet will be left to the Soviet Union, which in the last two weeks has upgraded its mission to Halley and made it more international in response to the U.S. move.

"We see this as a reverse trend of what used to be the case," said European Space Agency chief scientist Jacques Blamont yesterday. "The Russians now appear as a promoter of international cooperation, the Americans . . . as a demoter of cooperation."

Not only has the Soviet Union agreed to fly one of its spacecraft as close as 1,000 miles from the head of Halley's comet, it has agreed to have the craft carry an improved camera to be built by France, Hungary and Czechoslovakia. The Soviets are also understood to have invited West Germany to design an instrument for the mission -- the first such overture to the West Germans.

The proposed cut in the space budget that angers the West Europeans the most is the decision to eliminate one of two spacecraft that were to be flown over the north and south poles of the sun in 1986. No fewer than 17 Europeans instituted have spent $15 million on instruments that would have been carried by the craft.

The Europeans contend that the entire concept of the so-called "solar-polar" mission is threatened by the elminiation of the two-spacecraft mission, on which the European Space Agency has spent $100 million. One spacecraft was to fly over the north pole of the sun, the other over the south pole.

"This investment was judged against anticipated scientific objectives, which would now be seriously degraded," said the aide-memoire.

No European country is more angered by the proposed budget cuts than France, which saw 90 percent of its cooperative space ventures with the United States jeopardized in the last two weeks. Not only would the proposed cuts wipe out all the French experiments on the solar-polar mission, they would eliminate the single French experiment on the Gamma Ray Observatory mission.