A brand of polio vaccine routinely given to children has been withdrawn over fears it could be contaminated with BSE.
The UK Department of Health has told doctors not to use the
Medeva brand of polio vaccine after it found the maker had breached CJD safety guidance.
But "hundreds of thousands" of doses of vaccine produced in breach of these guidelines have already been given to patients.
Calf foetuses from the UK were used to make the vaccine, and while officials say the vCJD risk to patients is "incalculably small", it is recalling all remaining stocks as a precaution.
Oral polio vaccine is routinely given to children - often in the form of a "sugarlump" - and travellers visiting certain countries.
Vaccine manufacturer Medeva had given assurances that none of its products had been made using beef tissue, in accordance with European guidance issued following of the BSE scare.
However, the Medicines Control Agency discovered that chemicals derived from cattle tissue had been used by Medeva to make this vaccine.
The agency has told doctors to reassure patients that the recall is purely a precaution, as the ingredient, foetal calf serum, has never yet been found to have a capacity to infect humans with vCJD, although there may be a "theoretical" risk.
In addition, they say, part of manufacturing process aims to wash off unwanted cattle proteins which may harbour any potentially infectious agents.
Chief Medical Officer Liam Donaldson wrote: "Taking these points together, the risk associated with Medeva oral polio vaccine is incalculably small.
"I know that this recall will worry parents, but it is important to remember
that polio is a potentially lethal disease which we have virtually eliminated
from this country.
"As late as 1955 there were nearly 4,000 cases in England and Wales. Only by
keeping our children vaccinated can we ensure it does not return to this
He said that scientists had worked out that only a single hundred millionth of a mililitre of bovine material could be possibly passed on in a dose of vaccine.
He said: "It's not something that is an active ingredient of the vaccine but tiny traces can be carried forward into the vaccine itself."
However, vaccine safety campaigners say the revelation should reawaken debate on the way they are manufactured.
Ann Coote, a spokeswoman for JABS, a self-help group for people worried about
vaccines, said: "This will scare parents. They will want to know: `Why weren't
we told before? What really is the risk?'
"The Government always says the risks are minimal, but we won't know, will
we, until and unless someone gets the human form of BSE from it."
GP Peter Skolar predicted chaos at GP surgeries as parents came to be reassured about the risks.
He said: "These are questions which in all honesty I can't answer."
The MCA has checked with other polio vaccine manufacturers, and found they were complying with the 1999 European guidance which advised against the use of cattle products from any country with a reported case of BSE.
SmithKline Beecham, which makes its own vaccine, says it has sufficient supplies to cover the shortfall, although there may be short term supply problems.