Spain's leading magazine has criticized the Reagan administration for what it suggested was a slow and lukewarm U.S. response to the abortive military coup last month.

A cover story entitled "Reagan Washed His Hands" in the latest issue of the magazine Cambio 16 said that by lagging well behind other Western governments in condemning the attempted Army takeover, the United States had assumed an abbivalent posture while awaiting the outcome.

U.S. Embassy officials termed assertions that the Reagan administration withheld support for democracy while the coup attempt was in progress as a "gross and malicious misinterpretation." The American Embassy, along with other foreign missions here, had appeared totally unprepared for the developments of Feb. 23 when rebel Civil Guards took over the parliament building.

It took several hours, an embassy official admitted, to establish what was happening and, for a time, the impression was that the assault had been carried out by left-wing Basque terrorits.

According to Cambio 16, the U.S. reaction could affect substantially relations between Madrid and Washington, which is Spain's sole military ally.

The magazine's statement was termed irresponsible by a Spanish government spokesman, but a senior member of the Socialist Party expressed his fear that a public perception of U.S. ambivalence toward Spanish democracy, compounded by the administration's developing policy in Central America, could encourage renewed military plotting. The Socialist executive added, "Now more than ever we need the conservatives to fully commit themselves to Spanish democracy."

The Spanish gripe centers on a statement on the coup attempt by Secretay of State Alexander M. Haig Jr. on the afternoon of Feb. 23. Haig said, "We have nothing to say other than to say that we have been following the situation as it has developed. It is still too early to make any comments. It is an internal matter."

U.S. officials here said that this was not a statement of policy by rather an informal response to a journalist's question posed to Haig as he was leaving a meeting with visiting French Foreign Minister Jean Francois-Poncet. The officials stressed that Haig did not wish to comment on what was occuring in Spain until he had the facts.

A spokesman at the Spanish Foreign Ministry said no official complaint had been lodged in Washington. However, he did not rule out the suggestion that in informal diplomatic contacts regret had been expressed over what was generally believed to to have been a less than positive and prompt U.S. response.

Felipe Gonzales, leader of the opposition Socialist Party, is reported to have bluntly expressed his concern over the U.S. position during a visit to London last week.

A major factor in the concern over the U.S. response is the widespread rumor circulating among politicians and journalists -- and apparently spread by the plotters, but without any substantiation -- that Washington had prior knowledge that Spain's fragile democracy was heading for a showdown with the military. This was flatly denied by a U.S. Embassy spokesman.

The spokesman said rumors of contacts between coup plotters and American officials were absurd and that they sounded as if the conspirators were seeking to generate support by involving the United States. cAccording to the rumors, throughout last fall plotters making contact with civilian and military individuals consistently maintained that the same plans for an eventual military-civilian junta were being passed on to the U.S. Embassy.

The day after the parliament takeover, with the coup attempt quashed, the State Department issued a congratulatory statement underlining the "strength shown by the institutions and leaders of Spain's new democracy." Reagan also telephoned King Juan Carlos in Madrid, expressing his support and satisfaction at the resolution of the crisis.

The consensus in Spanish political circles is that the reaction came too late. Haig wrote his congratulatory note to Foreign Minister Jose Pedro Perez-Llorca on Feb. 26, three days after the crisis broke, and Reagan wrote his letter to the kind on Feb. 27. In fact, the telexed advanced copy of the president's warm letter ("The world is inspired not only by the Spanish experiment in achieving peaceful, democratic progress but also by the spirit with which you confronted the outrageous conduct of those who tried to bring change by force") only reached the U.S. Embassy on March 3 and was forwarded to the Royal Palace on March 4.