Spain had decided to buy 48 French F1 Mirage warplanes in an $800 million deal, causing an indefinite postponement in plans to purchase 72 United States F16s from General Dynamics, diplomatic and industry sources said yesterday.

There was no immediate confirmation from the Spanish government, but sources said an announcement could be expected soon.

"We lost out to the French," Dynamics spokesman said here. "We've been advised not to make any attempt to reverse the decision."

For the past few weeks the American company has been pressing the Spanish Air Force and government for a commitment to purchase the 72 F16s for about 1.5 billion, but they opted instead for the French plane, sources said.

The Spanish government was apparently influenced by political as well as by economic considerations. Spain wants French support for Common Market entry. The plane purchase is likely to influence French attitudes about expanding the Euporean Economic Community to include Spain. The manufacture of plane parts in Spain called for in the Spanish-French deal will provide jobs for the country's small but depressed aircraft industry.

General Dynamics representatives apparently believed that they had an inside track because the 1976 U.S. Spanish bilateral treaty contains a weapons procurement agreement calling for Washington to help Spain obtain 72 F-16s, the plane favored by version of the French F-1.

Both the Dassault Mirage F1 and the General Dynamics F16 are supersonic, single engine planes that can be used either for aerial combat or attacking targets on the ground with bombs and rockets. The Pentagon has been pushing hard to make overseas sales of the F16. The F16 is slightly lighter and more maneuverable than the French plane.

Late last year, however, it became clear that Spain was backing away from purchasing the American planes when it canceled other plane procurement provisions in the treaty's complex weapons agreement.

Why the plane was mentioned in the treaty has never been explained. Spain officials contend the agreement was included at the insistence of former U.S. Secretary of State Henry Kissinger but U.S. officials deny this.

Spanish officials would neither confirm nor deny that the Spanish Cabinet has already discussed the agreement to acquire the F1. Sources said the contract with France calls for co-production of the plane in Spanish factories and includes the sales on an undisclosed number of French helicopters at a "bargain price" as a further incentive.

The commitment of $800 million for the F1 means that Spain will spend practically all its available warplane funds for the next two years on the figther. The Spanish Air Force plans to expand its complement of fighter planes to 300 by the mid-1980s.

It was the second time in recent years that Spanish backed away from ordering an American warplanes in favor of the F1. In April 1975 Spain canceled an order for 24 F4 Phantoms in favor of a deal for the Mirage jets.

The Spain air force already has 14 F1s, and 10 more are scheduled for delivery this summer. The purchase of 48 more for delivery by 1980 will give the air force two full wings of F1s.

General Dynamics and other U.S. warplane manufactures expect that Spain will turn to them for the next generation of tactical aircraft which will be ready for delivery in 1984-85. A Spanish Air Force mission is scheduled to go to the United States in the next two months to study American planes.

Spanish air force officers have been reported unhappy with their dearth of modern planes and equipment. Since 1969 - despite Spain's close relationship air bases on Spanish territory - Spain has balanced warplane acquisitions between the United States and Western Europe, with France getting most of the European purchases.