Blood can be contaminated with BSE
Officials issued a statement in response to comments by the head of the American Red Cross who said she would do all she could to avoid having a blood transfusion in the UK.
Bernadine Healy says a risk remains of contracting variant CJD from contaminated blood, despite controls and checks now in place.
The American Red Cross bans anyone who has visited Britain for more than three months in the past 20 years from donating blood.
Ms Healy told the BBC she wouldn't be prepared to take any chances.
"We do not know if there is a silent reservoir of people who are carrying this disease who may be blood donors.
"That is a theoretical risk, but it is a real theoretical risk."
She said that until a blood test could determine whether a donor was infected, it was sensible to take precautions.
"The only thing we can do is to wall off blood donors based on their potential exposure to beef that could be from a cow that has BSE."
The States is about to ban imports of blood from Europe and also donations from here because of BSE.
Catastrophe
However, the health authorities there have warned the Bush administration that the country faces a "catastrophe" if the ban goes ahead.
Kenneth Rasky, head of the New York Hospital Association, said: "If this policy were adopted that would knock out about a third of the blood supply in New York.
"That would force us to begin rationing services to our patients. Elective surgeries would be postponed, and possibly emergency services would also be affected."
Dr Rasky accepted there was a theoretical risk of vCJD infection from contaminated blood.
But he said: "It is a balancing act, and the clinical implications far outweigh the theoretical risk."
British response
The National Blood Service said it had taken precautions to safeguard blood donated in Britain.
Spokesman Chris Hartley said: "Her comments are her own views and she is entitled to them.
"We have erred on the side of caution against this unknown risk and taken various precautions."
He said a process known as leucodepletion removed the white cells from donated blood, as they are the ones thought to carry vCJD.
And plasma is imported - by coincidence from the United States - to ensure it is not contaminated with the disease either.
Professor Ian Franklin, medical director of the Scottish National Blood Transfusion Service, said Ms Healy's were very helpful.
"I think it is a pretty absolutist principle for someone who is 5,000 miles away and probably thinks that in this way they can somehow protect the USA from variant CJD or BSE.
"I think that is probably highly unlikely and the big worry for them is that patients in New York are going to run out of blood, possibly be denied cancer treatment and die, for a risk that may be theoretical."