The company that makes half the flu vaccine used in the United States said on Tuesday it will not supply any vaccine for the coming flu season because of problems at its plant in Britain, throwing a major U.S. flu drive into disarray.

British regulators suspended the manufacturing license of Chiron Corp., citing “problems with the manufacturing process.”

The U.S. Health and Human Services Department said this would pose a serious challenge just as the flu vaccination season gets under way.

“We currently anticipate having approximately 54 million doses of influenza vaccine from Aventis and about another 1 million to 2 million doses of FluMist nasal spray,” the department said in a statement.

FluMist is a new vaccine made by Medimmune and Wyeth Pharmaceuticals. It is given nasally and is not approved for those in most need of the flu vaccine - the old, the chronically ill, pregnant women and babies aged 6 months to 23 months.

This could mean a big scramble for the shot, which is recommended for 185 million Americans this year.

It takes months to make influenza vaccine, which is reformulated every year to match the strains that scientists believe are the most likely to make people sick each season.

Nonetheless the Health and Human Services Department said it was looking at possible ways to make more doses in time for this year. “This includes working with Aventis on its ability to provide more vaccine,” the department said.

Chiron had been expected to supply 48 million out of 100 million doses of vaccine this year, and federal health officials had already launched a campaign to urge Americans to get vaccinated against flu, which kills 36,000 people in the United States and 500,000 worldwide in an average season.

“This comes as a bombshell, I am afraid,” said Dr. William Schaffner of Vanderbilt University in Tennessee, a member of the National Foundation for Infectious Diseases and an organizer of the national influenza vaccine campaign.

“I think it is quite clear that almost half of the anticipated doses of influenza vaccine are not going to be available,” Schaffner added in a telephone interview.

“We are going to have to cope.”

This will mean convincing health-care providers, pharmacies, employers and others who already have flu vaccine to share it with others. Attempts have been made to do this in past crises, said Schaffner, and it has never gone smoothly.

Schaffner said the Advisory Committee on Immunization Practices, which helps set federal vaccine policy, was holding an emergency meeting on Tuesday.

Chiron had suspended shipment of its vaccine, saying it had some problems with purity. But only last week Chiron Chief Executive Officer Howard Pien assured a congressional hearing that his company had lost only 4 million doses and was on schedule to deliver 46 million to 48 million doses.

Aventis-Pasteur, the other major maker, says it has already shipped 30 million doses of vaccine and plans to produce a total of 52 million doses for this season.

“The fact that the U.S. will potentially only have 50 million doses is a public health nightmare,” said Bernstein & Co. analyst Geoffrey Porges.

Chiron’s flu vaccine is produced at a facility in Liverpool in Britain that is licensed by the U.S. Food and Drug Administration. About 90 percent of the vaccine produced there is shipped to the United States, the company said recently. Most of the rest goes to Britain.

“FDA officials are working with their British counterparts to learn more about the situation,” said Tony Jewell, a spokesman for the Department of Health and Human Services.

Health officials have complained for years that distribution of flu vaccine is uneven, with grocery stores often doling out immunizations while nursing homes go without. Nursing home populations are especially vulnerable to flu outbreaks and 90 percent of those who die from flu are elderly.

On top of that, last year’s early and unusual flu season frightened many people and health officials had hoped that would encourage more people to get vaccinated. The U.S. Centers for Disease Control said 152 children died of flu last year, many of them healthy.

Usually, fewer people get the vaccine than should and in many years vaccine makers have to throw away doses.