The Reagan administration was "deeply disturbed" by the attempted coup two weeks ago against the Philippine government of Corazon Aquino that would have had "disastrous" consequences for the island nation and for U.S.-Philippine relations, a senior State Department official testified yesterday.

In unusually strong language for a diplomat, Deputy Assistant Secretary of State David Lambertson told a House subcommittee that success of the Aug. 28-29 coup "would have marked a tragic reversal of fortune for that country and its people, destroying everything they had created through their courageous overthrow of {Ferdinand} Marcos and the restoration of democracy."

Lambertson expressed unequivocal U.S. support for Aquino and unequivocal opposition to her overthrow by elements of the military. These sentiments were endorsed in a resolution introduced in both houses of Congress and unanimously adopted by the House Foreign Affairs subcommittee on Asian and Pacific Affairs during Lambertson's appearance before it.

Subcommittee Chairman Stephen J. Solarz (D-N.Y.) said the congressional resolution was intended "as a very clear and strong message to people in the Philippines, especially the military, that a coup would have catastrophic consequences for relations between our two countries."

Private comments of both executive branch officials and lawmakers indicated a high degree of continuing concern about Aquino's leadership. Her entire cabinet submitted resignations on Wednesday so that she would have the flexibility to reshape her embattled administration.

A source familiar with U.S. official assessments said confidence in Aquino's survival in office has been shaken more than at any time since she took power in a "people's revolution" against Marcos in February 1986. Many officials believe that a strong assertion of authority by Aquino or an even more serious military challenge to her presidency may occur within the coming weeks, the source said.

Asked about the prospects for another attempted military takeover, Lambertson said "it can't be ruled out" because the leader of the recent mutiny, Col. Gregorio Honasan, remains at large.

Lambertson expressed satisfaction, however, that the recent coup attempt had won "no public support" and that by U.S. estimates no more than 2 percent of the Philippine armed forces had supported it at any time.

Aquino "now faces, in starker terms than previously, the formidable challenge" of mending the rifts within the military and the strains on civilian-military relations, Lambertson said.

In response to questions, Lambertson said he has seen nothing to substantiate rumors in the Philippines that American private citizens, including retired U.S. Army general John K. Singlaub, had a hand in the coup attempt.

Lambertson said any future coup attempts against Aquino, even if they fail, would weaken the Philippine body politic, adversely affect potential foreign investment and divert attention from fighting the communist-led insurgency, which he described as the most serious military problem facing the country. While the recent coup was undoubtedly "unsettling" to new investors, he said, indications are that it has not deterred those who had planned to invest in the Philippines.

The administration will "reexamine" U.S. aid to the Philippines to see whether more can be provided, Lambertson said. But in view of the foreign-aid budget problem, he made no promises.