Despite a new law and a series of reassuring statements, Philippine President Ferdinand Marcos is finding that uncertainties about his eventual succession die hard.

In a bid to lay the questions to rest, Marcos last month signed a hastily passed law formally bequeathing his powers to a committee to which he recently appointed his wife and eldest daughter. But the move apparently did little to remove doubts among Filipinos as well as U.S. officials, who long have encouraged measures for a peaceful transition of power in their most important ally in Southeast Asia.

After years of one-man rule under martial law and a series of constitutional changes that destroyed the previous succession mechanism, the new law finally gave the Philippines an agreed transition system shortly before Marcos, 65, made a rare state visit to the United States.

The continuing lack of any single, clearly designated or undisputed successor suggests the prospect of a power play should Marcos not complete his new six-year presidential term ending in 1987, according to diplomatic and Philippine observers here.

The issue keeps cropping up here despite government denials of rumors -- some of them spread by his political opposition -- that Marcos has serious health problems. Also serving to keep the issue alive has been the steady accumulation of power in the hands of his ambitious wife, Imelda

At the core of the uncertainties over the succession is that with 17 years in power, Marcos is the only president nearly half of the Philippine population has ever known. So completely has he dominated the political scene in the last 10 years -- eight of them under martial law -- that it is hard to imagine anything but confusion after his demise, the new succession mechanism notwithstanding.

Marcos signed the succession bill on his birthday Sept. 11 after prodding the National Assembly to resolve a debate and pass it before his state visit to Washington Sept. 15 through 21. The new law assigns the powers of the presidency to a recently formed Executive Committee in the event the president dies, becomes incapacitated, resigns or is deposed. The committee is then required to help organize a new presidential election within 90 days.

Nevertheless, questions about the succession and constitutional issues raised by the new law have continued to simmer, surfacing several times in the local news media while Marcos was away. He remained in the United States for a week after his official state visit ended.

In a recent Newsweek interview, Marcos said he had decided to create the 15-seat Executive Committee after consulting various Philippine leaders. "Modesty aside," Marcos said, "the conclusion they arrived at was: 'No single leader can take your place .' "

Under sharp questioning by reporters in Washington, Marcos appeared to rule out the accession of his wife to the presidency after he leaves office, but he said his eventual successor would probably need her "help."

Imelda Marcos, a 52-year-old former beauty queen whose power far exceeds the authority vested in her official positions, showed her preeminence in the Marcos government by signing a number of agreements with American officials during the U.S. state visit. Prime Minister Cesar Virata clearly took a back seat to her during the trip.

In addition to her posts as minister for human settlements, governor of Metro Manila and a member of the National Assembly, Imelda Marcos was appointed in August to the Executive Committee chaired by Virata.

Marcos' 26-year-old daughter, Imee, who scandalized the first family earlier this year by marrying a divorced Philippine sportsman, was named to the Executive Committee as an observer in her capacity as head of the National Youth Movement.

The appointment of Imelda Marcos to the committee is considered likely to lead to an erosion of Prime Minister Virata's powers, diplomatic observers said. Already she has begun to exert her influence in the body, they said. In a television interview broadcast here last month, however, the First Lady indicated she did not want to become president.

"The reason for this -- and it is a very selfish reason -- is because I do not want to lose my mystique as a woman," she said.

Despite her denials, "The general wisdom around town is that she very much wants to be president," a Western diplomat said. "In the last five years she has moved much more firmly in her own right to acquire authority" and has become "extremely prominent in the past year," he said.

"A whole new power structure is emerging since Imelda got on the Executive Committee," another diplomat said.

In addition to her usual duties, Imelda Marcos has played a major role in export promotion and has tirelessly pushed a new National Livelihood Program -- known locally by its Tagalog language initials as the KKK -- designed to create jobs in the economically depressed provinces.

She has also led negotiations for major deals with foreign countries, including oil purchase agreements with Iraq and Mexico, the sale of coconut oil to the Soviet Union and the construction by the Soviets of a cement plant in the Philippines. Furthermore, she has argued the Philippine position with the leaders of Saudi Arabia and Libya on the sensitive issue of the government's war against Moslem rebels in the south.

According to diplomats, a measure of her ambition surfaced last year when she made a determined bid to become prime minister, only to be turned down by Marcos in favor of the technocratic finance minister, Virata.

Imelda Marcos herself is credited with a shrewd political sense and some success in building a base of her own with local government officials around the country.

"Under existing conditions, she has a lot of people around the country who owe her one," a diplomat said. But whether they would still feel beholden if her husband were not around remains an open question, the source said.

Also debatable is the attitude of the military if she were to succeed Marcos, observers said. Further clouding this question is her reported rivalry with Defense Minister Juan Ponce Enrile, who is often mentioned as another potential successor to Marcos.

While Imelda Marcos ranks as the favorite at this point to succeed her husband, she still must contend with the perception of her among many Filipinos as a Marie Antoinette figure known mainly for her extravagance and promotion of lavish construction projects in a land where nearly half the population lives in poverty.