The BBC's Washington Correspondent Stephen Sackur : "US has banned blood donations"

The ban reflects concern about the possibility that mad cow disease might be transmitted to humans in the form of CJD through tainted blood.

In the case of the US, it will cut the blood supply by two or three per cent at a time when the blood is desperately needed, but officials are clearly determined to play safe.

The average tourist who spent just a few weeks in Britain can still donate blood - but people who went to Britain repeatedly between 1980 and 1997 will have to add up their trips to see if their total time amounts to more than six months in the UK.

Hema-Quebec, a Quebec based blood agency, says it intends to stop taking donations from those who have spent a total of just one month in Britain. The agency says fewer of its donors have visited Britain so it can afford the more stringent regulations.

UK blood supply is safe, say ministers

There is no evidence that mad cow disease, BSE, can be transmitted through blood - but some experts continue to believe it is a theoretical possibility.

Given the time lag between contracting the disease and the onset of symptoms, some American scientists say the worst effects of the disease have yet to be seen.

Medical researcher Peter Lurie, one of the doctors advising the US Food and Drug Administration, said: "We are dealing with a disease that may still be on the increase in terms of human disease, we are dealing with an organism that we cannot identify, and we are dealing with a disease that only causes symptoms several years after it is first transmitted.

"All of those things argue in favour of more caution."

Other countries said to be considering following the US and Canada lead on a blood ban include Australia and Japan.

UK insists blood is safe

The British government has repeatedly insisted that although there is a theoretical risk, the UK blood supply is safe.

Deputy Chief Medical Officer Dr Jeremy Metters said: "In theory there is a risk. We accepted that 18 months ago and we decided that we would take the white cells that are thought to be the problem out of donated blood."

Dr Jeremy Metters has insisted UK blood supplies are safe

Dr Metters said any theoretical risk was far outweighed by the serious risk to life that would result from refusing a blood transfusion where doctors recommended it.

He said: "If a patient is recommended to have a blood transfusion they should not hesitate because of this very theoretical risk."

BSE swept through British herds in the mid-1980s. Health and agriculture officials said it was harmless to humans until cases of the human version of the disease, variant CJD, started to be found.

The incurable and fatal disease is caused by a misshapen protein known as a prion, which has been found in a range of tissue, including the brain, lymph nodes and spleen, and has also been traced to the blood.