Reports that President Ferdinand Marcos is more seriously ill than the government admits are spurring new efforts among opposition leaders to unify their divided ranks and come up with contingency plans for a sudden transfer of power.

Although still unconfirmed, the reports also are prompting senior officials in Marcos' ruling party and the armed forces to consider the prospects of a transition to a post-Marcos era, according to government and opposition sources.

But with the government and military plagued by the same kind of rivalries and factionalism that beset the political opposition, no guarantees appear to exist that the untested constitutional transition process would be followed and that a scramble for power would not develop within and among the competing groups, Philippine political analysts said.

Further complicating the situation, according to leading members of both the government and opposition, are indications that the president's wife, Imelda Marcos, has not given up her ambitions to succeed him despite the political setbacks she has suffered this year.

"If Marcos disappears, she'll make a grab for it," said one opposition figure. "She cannot afford not to. She has so much to lose."

While preparations for a transition may be premature, they reflect widespread incredulity about the official line that Marcos, 67, was merely stricken with influenza when he went into seclusion on Nov. 14. He reemerged Nov. 26 for a televised budget-signing ceremony attended by members of his Cabinet that apparently was aimed at scotching rumors of serious health problems. Since then, his official activities have been limited.

Despite denials from his Malacanang presidential palace, the belief persists among some western diplomats and Cabinet officials that Marcos underwent some kind of surgery on or about Nov. 14.

Marcos presided over his first Cabinet meeting in several weeks on Saturday, according to the state-run television, United Press International reported. Agence France-Presse added that the president took his shirt off to show Cabinet members that he had not undergone any surgery, according to the Philippines News Agency. There was no independent confirmation of the agency's report.

Perhaps more remarkable -- and, to some, alarming -- is that even long-serving senior government officials do not seem to know what transpired. So secretive has Marcos become about his health that even such officials as his designated transitional successor, the speaker of the legislature, Nicanor Yniguez, and Defense Minister Juan Ponce Enrile were left guessing about his condition.

Much of the medical speculation centered on heart or kidney problems. Marcos long has been reliably reported to suffer from lupus erythematosus, a degenerative kidney disease that can affect other vital organs.

The uncertainty has prompted opposition leaders to hold a series of meetings recently to seek unity in the event of a presidential election before 1987, when Marcos' current six-year term expires.

Under a constitutional amendment passed in January to establish a succession process until 1987, the legislature would meet three days after Marcos died or left office and would pass a special election law within a week of the meeting. Election of a president and vice president would then take place within 45 to 60 days. Meanwhile, the speaker would serve as an interim chief executive with limited powers.

Starting with the 1987 election, the vice presidency -- abolished by Marcos when he ruled the country under martial law from 1972 to 1981 -- is to be restored to take care of the succession question.

While a number of opposition preparatory committees and transition measures have been discussed among Marcos' foes to wrest power in the event of a presidential vacancy, no consensus has emerged yet on a common candidate. Opposition leaders acknowledge that such unity is vital to beat Marcos' ruling New Society Movement in an election, but the assassination last year of former senator Benigno Aquino Jr. deprived the opposition of its only unifying figure.

Among those now mentioned as possible opposition presidential candidates are Salvador Laurel, the head of the United Nationalist Democratic Organization; Jose Diokno, a former senator and leading human rights activist; Aquilino Pimentel, leader of the Philippine Democratic Party-Laban group; Eva Estrada-Kalaw, a leader of the Liberal Party; Lorenzo Tanada, the 86-year-old patriarch of opposition politicians; Raul Manglapus, a foe of Marcos living in exile in Washington; Jovito Salonga, another exile based in California; Rafael Salas, a senior Philippine official at the United Nations, and Ramon Mitra, a newly elected member of the legislature.

Also prominently mentioned here are two Aquino relatives: Agapito (Butz) Aquino, the younger brother of the assassinated opposition leader, and Corazon Aquino, his widow.

A major problem in achieving unity, opposition politicians said, is that so many Marcos foes think they can win a presidential election because of perceived popular dissatisfaction with the government.

No less potentially fractious, however, are the contenders in the government camp. Besides Imelda Marcos, those most prominently mentioned as possible successors include Prime Minister Cesar Virata, Defense Minister Enrile, Labor Minister Blas Ople and Deputy Prime Minister Jose Rono.

In an interview, Enrile said that he still expected Marcos to make good his pledge to run for president again in 1987 and that he would support Imelda Marcos if she were nominated by the party.

Enrile acknowledged, however, that he "might" become a presidential candidate himself if he felt this was necessary "to control a difficult situation." Asserting that "we are not lacking for talent in this country," Enrile chided President Reagan's suggestion during his last debate with Walter Mondale that the only alternative to Marcos was a Communist takeover.

"If he Reagan had had all the facts at hand about the Philippine political situation, he would not have said that," Enrile said.

For its part, the left also apparently has been reacting to the uncertainty about Marcos' health by promoting the concept of a "democratic coalition government" to replace him. Openly endorsing positions of the Communist Party of the Philippines and its armed wing, the New People's Army, a main leftist grouping called the Nationalist Alliance for Justice, Freedom and Democracy has called for the "dismantling of the U.S.-Marcos dictatorship" through a combination of nonviolent and armed struggles and for its replacement by a "people's democratic republic."

Enrile charged that the Nationalist Alliance was a Communist front organization and that the radical left was increasingly taking over from political moderates in the group.

While the prospect of a chaotic transition reportedly has worried some senior military officers, Enrile stressed that there was widespread agreement on the need "to maintain the sanctity of the constitution," and he dismissed the idea of any coup attempt.

Nevertheless, opposition politicians noted, Marcos' illness comes at a time when the military also is in a transitional stage.

The armed forces chief of staff, Gen. Fabian Ver, was temporarily relieved of command in October after he and 24 other military officers and men and one civilian were implicated in the assassination of Aquino. Ver's longtime rival, Lt. Gen. Fidel Ramos, was appointed acting chief of staff.