The leader of a major Philippine opposition group threatened today to boycott all future elections unless President Ferdinand E. Marcos resigns and a caretaker government acts to assure fair elections and a new constitution.

Salvador H. Laurel, whose United Nationalist Democratic Organization (UNIDO) is a coalition of leading groups opposed to Marcos, said that such a radical change is necessary because of widespread public skepticism over circumstances of the slaying of longtime Marcos foe Benigno S. Aquino Jr.

Demands that Marcos resign have become commonplace in the wake of Aquino's assassination more than a week ago, but Laurel is the first significant leader to threaten abandoning opposition politics if the president does not step down.

For three weeks there has been speculation that Marcos, who has been in power for 18 years, is ill, and that a struggle for succession is underway within his inner circle.

But there are no indications that a resignation is imminent, and diplomatic observers here regard as highly improbable the idea of Marcos naming a caretaker government. Most likely, they say, Marcos would be replaced by a military leader, his influential wife Imelda or some other trusted intimate.

Laurel's ultimatum was taken here, in part, as a sign of desperation among the opposition, rendered more leaderless than usual by Aquino's murder.

"Aquino was a shrewd strategist who had the talent to weld together the disparate elements of the opposition," one well-informed Western diplomat said today. "The question today is whether the opposition can now find a voice."

Laurel is as close to a united opposition leader as anyone here. UNIDO is an umbrella group embracing 12 political organizations opposed to Marcos.

At least one other opposition figure, Jose Diokno, a Democratic Socialist and former senator, took roughly the same position in a recent interview as Laurel did today in a speech to the Manila Rotary Club. Diokno, whose organization does not belong to UNIDO, said that elections are useless as long as the Marcos government controls the voting process. Past elections have provoked charges of vote rigging by Marcos' forces.

An election for 180 seats in the National Assembly is scheduled for next May, and Marcos' current term as president does not expire until 1987.

Laurel laid down four conditions for his proposed caretaker government--a truthful investigation of the Aquino assassination, amnesty for all political prisoners, a new constitution and a general election for both the presidency and National Assembly seats to be conducted in a "free, orderly and honest" manner.

"Only thus will we achieve national unity or national reconciliation," he said. "Only thus will we ever again participate in any election."

Speculation that Marcos was ill and possibly thinking of stepping down intensified early last month, when he announced that he would go into seclusion to write a book. He was rarely seen in public until the night after Aquino's assassination, when he went on television to assure listeners that the government was not involved in the killing.

That night he proclaimed himself healthy and ready to "wrestle" anyone in the room, filled with reporters and government officials.

Rumors persist, however, that Marcos is suffering from a severe skin disease and a malfunctioning kidney. Some who have followed his career say that his face is puffier than usual and that his speech is often slurred.

But these reports were disputed today by a Western diplomat who has seen and talked by telephone with Marcos recently. "I cannot perceive any notable differences in his appearance in the past year and a half," the diplomat said.

When the question of presidential succession comes up, informed observers here say that two factions within the inner circle seem to be in contention. One is led by Gen. Fabian Ver, chief of staff of the armed forces who is said to have the confidence of Imelda Marcos. The other faction is represented by Defense Minister Juan Ponce Enrile and the vice chief of staff of the armed forces, Lt. Gen. Fidel V. Ramos.

Observers here say that Ver's star seems to be rising. On Aug. 1, shortly before Marcos began his period of seclusion, he announced a shakeup in the line of command that gave Ver direct control over the integrated National Police, previously controlled by Ramos.

Although there are no signs now that Marcos could be ousted or that he would abdicate, many independent analysts feel that he must make some drastic moves to satisfy a public that is widely suspicious of the government's role in the Aquino case.

"Marcos is regarded as one of the shrewdest politicians in this part of the world," said one diplomat, who is closely acquainted with Marcos. "But no politician of his class could fail to address a national problem of such proportions."