The Reagan administration has decided to allow private satellite companies to keep secret some data collected by satellites and sell it under exclusive contract to a single company or country.

The decision affects a 25-year-old policy under which satellite pictures of the Earth--as well as oil and mineral data--have been freely available to all nations.

The satellite flights have long been controversial, with some nations contending that the photography was "spying" and that the information could be used against them by other nations or multinational companies.

The United States has successfully countered the argument by making all its land sensing satellite pictures available, cheaply, to all nations so no country can get an edge over another.

This open sharing of satellite data has been the linchpin of U.S. satellite policy. It was reaffirmed annually when this country went to the United Nations to argue for and vote for the "open skies" policy and unrestricted sharing of land satellite data.

The "open skies" policy states that all countries are free to put up satellites, and that all the data collected should be sold to all comers on a nondiscriminatory basis.

Lisle Rose, an official in the State Department's Office of Science and Technology, said the action by the department is "not a change in policy yet" but may or may not become one when private companies begin to operate.

Rep. James H. Scheuer (D-N.Y.) said that many nations have expressed serious concern about the apparent change in policy.

"The United States has gone to great difficulty to assure countries, especially underdeveloped countries and many of them our strong allies, that the data is freely and openly to be used . . . . They have a very legitimate fear . . . that if satellites are to operate without restrictions, it becomes the robber baron concept of space spying, with the higher bidder having access to their own resources when they themselves do not," Scheuer said.

As an example, he cited Nigeria, a country with possibly undiscovered oil resources. If a corporation could get exclusive satellite information about Nigerian oil, it could use satellite data "just like a secret geological report to negotiate a favorable deal for themselves," he said.

Scheuer also said that if the policy were allowed to go ahead unchecked, it would create "a very strong international backlash . . . and have a very detrimental effect on our international relations, of that there is no question."

The Canadians, British, and Indians have already raised serious questions, and other countries such as Thailand and Nigeria may feel threatened by the policy, Scheuer said.

The State Department action came as the Reagan administration prepares to sell the nation's land and weather satellites to private companies. The department determined that it would not require any private company to abide by the "open skies" policy. Any private owner, however, would be "encouraged to conform . . . as closely as is commercially possible to traditional U.S. policy . . . , " the department said.

State Department officials said the reason for stepping back from the policy was to give private companies that want to go into the land-satellite business a competitive edge over foreign competition.

The only potential competitor now on the horizon is a French-government-supported group, the Spot Image company. That company has declared that it will abide by "open skies" completely.

Eight weeks ago, at a meeting of the Landsat Advisory Committee of the Commerce Department, the State Department announced that it will not require U.S. corporations to follow the "open access to data" rule.

"I was rather amazed . . . as someone put it, my teeth nearly fell right out of my mouth," said Murray Strome, an official of the Canada Center for Remote Sensing.

In a General Accounting Office report requested by Rep. Jack Brooks (D-Tex.), expected to be released at congressional hearings today, several other nations are strongly critical of the new policy, according to congressional aides.