One patient died after receiving blood from a vCJD infected donor

As part of efforts to halt the spread of vCJD, the human form of BSE, Britain has sourced all plasma for blood products from the US since 1999.

The Conservatives are now calling on the Food Standards Agency to make checks to ensure blood remains safe.

There is no proof yet that vCJD can be transmitted through blood transfusions.

But the potential danger was highlighted last week when it emerged a patient in a British hospital had died from vCJD after receiving blood from an infected donor.

Tory health spokesman Andrew Lansley said Britain could not halt imports of US blood products as it would almost certainly be impossible to find sufficient supplies from a safer source.

But he is calling for action to reassure the public of the safety of the US supplies.

BOVINE SPONGIFORM ENCEPHALOPATHY First surfaced mid-1980s Can pass to humans through infected beef products Human form of disease called vCJD vCJD has killed 137 people, mainly in the UK US searches for 'mad cow' clues

"There is an important issue that arises from this case, which is to try to maintain the integrity of the American food chain, so we can be sure that American blood products continue not to be at risk of contamination from BSE, leading to vCJD," Mr Lansley said.

"Therefore, the two things I am looking for when Parliament returns [on 5 January] are firstly, for us to hear from the FSA that they are working with the Food and Drug Administration in America to ensure inspection and controls in America are as good as anywhere, and secondly, to hear from the health secretary [John Reid] what additional questions he is asking to make sure American blood products are as safe as we can make them."

Several countries banned US beef imports hours after America announced its first suspected case of BSE.

The move was led by Japan, which bought $800m of US beef last year, and Canada later followed with a partial ban.

COUNTRIES THAT HAVE BANNED US BEEF Japan Canada - partial ban South Korea Singapore Taiwan Malaysia Thailand Hong Kong Russia Australia South Africa Brazil Mexico Ukraine China

Mexico, Brazil, Russia, Ukraine, and a number of Asian counties from Singapore to South Korea were among those who followed Japan's example.

Early on Thursday, China became the latest country to suspend beef products from the US.

The European Union said it was keeping a close eye on the situation, but it has anyway banned most US beef for many years because of growth hormones in the meat.

Tissue samples from the suspected cow are already being studied in the UK, which suffered a devastating outbreak of BSE in the mid-1990s.

The results will be known in a few days' time.

So far, 143 cases of vCJD have been diagnosed in the UK, although the numbers of new cases are falling.

There is no established treatment for the illness, which causes massive brain damage and normally kills within months of being detected.