Some Americans who traveled frequently to Britain during that country's mad cow disease crisis are being banned from donating blood back home--a restriction that will cut the U.S. blood supply during a critical time of shortage but one the government deems a necessary precaution.

The Food and Drug Administration imposed the ban yesterday on blood donations by anyone who has traveled to, or lived in, Britain for a total of six months since 1980.

The average tourist who spent just a few weeks in Britain may still donate blood--but people who went to Britain repeatedly between 1980 and 1997, the crisis years, will have to add up their trips to see if they're under the six-month limit.

Canada issued a similar restriction yesterday.

The donor ban is strictly a precaution; there is no evidence that any mad cow-type illness has been spread through blood transfusions.

But the mad cow disease that swept through Britain's cattle has been linked to a human brain-destroying illness, and both illnesses are so mysterious that scientists simply can't rule out the possibility they could infect blood.

Still, the FDA's donor ban is controversial. It is sure to frighten Americans whose blood is refused.

Worse, the American Red Cross estimates that the ban will cut U.S. blood donations by 2.2 percent at a critical time. Even before yesterday's action, experts were predicting severe, nationwide blood shortages as early as next year because of falling blood donations.

As many as 200,000 to 250,000 blood donors could be turned away by the new ban, said James MacPherson of America's Blood Centers, which represents blood banks that provide half the nation's supply. Because people in the Northeast and large cities travel more than people in other parts of the country, areas such as New York could be hard hit.

The FDA plans to work with blood banks to find ways to ease the donor loss, but MacPherson said the government also needs to help blood banks explain the issue to the confused or frightened donors they turn away.

At issue is an infection that kills by literally eating brain tissue and creating holes. In cattle, it's called mad cow disease, and it swept through British herds starting in the late 1980s.

About one in 1 million people around the world gets a similar brain disease called Creutzfeldt-Jakob disease, or CJD. Although CJD sometimes is hereditary, usually its cause is not known.

The worry about blood stems from Britain's discovery in the mid-1990s that some people apparently caught a new strain of CJD by eating beef infected with mad cow disease. Named "new variant CJD," it has killed 41 Britons.