Frank Dobson, Secretary of State for Health, admitted to the Commons that there was a "theoretical" risk from the Anti-D immunoglobulin injection, given to about 80,000 pregnant women in Britain each year to prevent their babies developing a potentially fatal form of anaemia.
The Anti-D supplies now in use came from British donors, and so could theoretically could be contaminated with the "prion" that causes the fatal "new variant" Creutzfeldt-Jakob disease (nv-CJD), the human form of BSE.
Four of the 31 people who have so far died of nv-CJD were blood donors, and studies suggest that the prions can be carried by white blood cells. There is no test to detect prions in blood, and symptoms may not appear for decades after infection.
However, Mr Dobson said there was a worldwide shortage of Anti-D immunoglobulin, and it will take a few months to introduce fresh supplies from the United States.
The Government decided in February that British donors should not be used for a range of blood products which involve white cells, a move that will cost pounds 100m annually.
But the National Blood Service said it had taken until now to find suitable sources free of the HIV and hepatitis viruses. And a spokeswoman admitted that it could take months to acquire non-British supplies of Anti-D, which has to be made specially by inoculating men with foreign blood cells.
The Department of Health said last night: "The CJD risk remains theoretical, but the risk to babies without Anti-D is known.
"In our view, the balance of risk and benefits is far greater towards Anti-D than the theoretical risk of nv-CJD."
The Tory health spokesman Alan Duncan urged Mr Dobson to issue a list of all products at risk of contamination.
"Patients need to know the risks and they need to exercise an informed choice," he said.Reuse content