ASHINGTON, Nov. 15 — The Bush administration, reversing a course set two decades ago, has decided that the world's remaining stocks of smallpox should be retained until scientists develop new vaccines and treatments for the disease, a process that could take years if not decades.

The decision, disclosed by senior administration officials, is likely to provoke criticism from international health officials who have long favored the destruction of the microbe.

A succession of administrations have endorsed the goal of destroying the virus, which was eradicated as a disease in the 1970's. But some American scientists and Pentagon officials have argued for retaining smallpox stocks, and in 1999 President Bill Clinton declared that they should be maintained, at least temporarily, while more research was conducted.

The Clinton administration privately assured other nations that it would support a move to kill off smallpox in 2002 when the issue was considered by the World Health Organization, which has long advocated destruction of the virus.

The Bush administration's new policy, which is now being described to America's allies, sets no such deadline and establishes some stringent conditions, reflecting a new assessment of the dangers posed by bioterrorism.

The United States stopped routine vaccinations for smallpox in 1972. How long vaccines continue to protect against the disease is not known but the immunity is believed to fade over time. Americans under 30 are completely vulnerable to the disease.

Administration officials said the remaining American smallpox samples, which are stored at a laboratory at the Centers for Disease Control and Prevention in Atlanta, should not be destroyed until the nation develops at least two licensed antiviral drugs, a vaccine that can be taken by the entire population, and other defensive measures. Russia also has smallpox strains stored at a research laboratory in Siberia.

There is currently no treatment for smallpox and a new vaccine is under development. Experts said today that work on both would most likely take many years to complete.

The eradication of smallpox as a disease is considered one of medicine's greatest triumphs and experts said the Bush administration's decision is likely to anger many doctors and scientists, particularly those in developing nations ravaged by the disease only a quarter of a century ago.

But administration officials said that after the terrorist attacks of Sept. 11 and the spate of anthrax letters that have killed four, infected 13 others, and put 30,000 Americans on antibiotics, the administration had no choice but to abandon the nation's longstanding commitment to eradicating the officially declared stocks as soon as possible.

"The issue was straightforward," said a senior official. "Are we going to do what we can to be prepared for what is one of the most consequential threats we face, or are we going to engage in feel-good measures that mask the real danger?"

Officials said that an interagency group that has been considering bio- defense measures had unanimously endorsed the policy shift without reservation. The group is made up of representatives of the Defense Department, State Department, Department of Health and Human Services and other cabinet departments. But the officials said they expected the decision might be criticized by individuals and groups long associated with the international campaign to destroy the virus to ensure that it does not re-emerge and to celebrate the world's triumph over the contagious disease, which killed one-third of those it infected.

Jonathan B. Tucker, a bioterrorism expert at the Washington office of the Monterey Institute of International Studies and the author of "Scourge: The Once and Future Threat of Smallpox," said the policy would create a certain amount of ill will internationally and arouse suspicions about American and Russian intentions.

In particular, he said, India and Brazil, which had lobbied hard for the virus's destruction, would resent the continued American and Russian monopoly on the stocks. The Clinton administration had assured their officials in 1999, he said, that the United States would support destruction after research on the virus was completed in 2002. It was that compromise that permitted the agreement two years ago, Mr. Tucker said.

The new policy, however, is likely to be received with relief by many countries, notably Russia. Russia has vigorously argued that there are clandestine stocks of smallpox virus throughout the world and that retaining the virus could speed the development of new drugs to fight a possible outbreak, whether because of terrorism or other factors.

Reached late tonight by phone in Moscow, Lev S. Sandakhchiev, the director of Russia's State Research Center of Virology and Biotechnology, the Siberian-based research laboratory where smallpox strains are stored, called the American shift wise not only for Russia and the United States, but also for the entire world.

In the Soviet era, Dr. Sandakhchiev's lab specialized in turning viruses like smallpox into weapons of war. American officials said that he had privately warned that North Korea, among other countries, was secretly keeping smallpox stocks.

It was intelligence from Soviet defectors and other sources about Moscow's vast germ warfare program that prompted the Clinton administration to question the immediate destruction of the virus. Western and foreign officials said at the time that fear of a possible epidemic in a now largely unvaccinated world helped shift Mr. Clinton's personal views against immediate destruction.

But the previous administration remained committed in principle to destroying the virus, a commitment that the Bush administration has now refused to make.

Those familiar with the conditions set by the Bush administration say destruction of the virus will not even be considered until a new vaccine is produced that can be given to all Americans. The vaccine available today cannot safely be administered to people with H.I.V. and others with immune deficiencies.

The conditions also include the development of reliable medical diagnostic tests and environmental detectors, which do not yet exist. And they include the ability to defeat genetically altered strains of smallpox.

It would take at least a decade to meet the administration's criteria, and that is a conservative estimate, Mr. Tucker said.

He called the position of the administration too open-ended and warned that it would have its work cut out in trying to persuade other nations to accept that the virus would not be destroyed by the time President Bush leaves office.

A committee of scientific experts is scheduled to consider the issue of destroying the virus early next month. The new American position is expected to be unveiled formally at that session, if not before, officials said.

Administration officials said that they had already discussed the new stance with Britain, America's closest ally in the war against international terrorism, and that other allies were now being notified. Over the weekend, officials said, Tommy G. Thompson, the Secretary of Health and Human Services, discussed the policy by phone with the head of the World Health Organization.

Its officials, in Geneva, could not be reached late tonight for comment.