BBC News Online looks at the reasons behind the decision.

What has been decided?

Fresh frozen plasma, the fluid in which the red and white cells and platelets are carried around the body, is to be imported from America.

It contains clotting factors, antibodies, albumin and minerals.

It will be used to treat new-born babies and children born after 1 January 1996.

The plasma will also receive a further treatment which can kill of other blood-borne viruses, though not vCJD, to reduce risk.

Why is the step being taken?

There are concerns that new variant CJD could be passed on via infected blood. No humans have caught the virus in that way.

But a study recently showed one in six animals given blood from sheep infected with a similar disease went on to develop the illness, suggesting that there was an "appreciable" risk to humans.

The Department of Health stresses it has made its decision as a precaution after experts advised about the theoretical risk of transmission.

It says it is not a reaction to the research in animals.

Why has the Department of Health turned to the US for its supplies?

There have been no cases of BSE in the US, and the few individuals who have developed vCJD did so after being exposed to it in Britain.

The National Blood Service said there is no test which would pick up the presence of vCJD in blood. So all they can do is reduce the risk of transmission as much as possible.

The theory is that in Britain people have been potentially exposed to vCJD through the food chain, whereas in the US, people have not. Therefore, the risk of a blood donor being infected with vCJD in the US is less than in the UK.

The US plasma will be obtained from single donations from voluntary, unpaid, donors.

Why does the new rule only apply to children under six?

It is all about minimising the risk of vCJD transmission.

Children born after 1 January 1996 should not have been exposed to vCJD from the food chain because of rules introduced then which banned feeding cattle meat and bone meal.

Deputy Chief Medical Officer Dr Pat Troop said it was mainly very young children who underwent treatment in which they would need the plasma.

It is used in a range of procedures for premature babies and babies and children who have had heart surgery, liver transplants or major accidents and injuries.

What safety measures are there for children over six and adults?

Since 1999, plasma has been imported to make clotting agents such as Factor 8, Factor 9 and albumin, to treat haemophiliacs and patients who have clotting problems after surgery.

Other safety measures include not taking blood from high-risk donors, and testing all units of blood for the presence of infections such as HIV, hepatitis B and hepatitis C.

What happens next?

The National Blood Service will now start the tendering process, which will take nine to 12 months, for England and Wales.

The Scottish Blood Service is also negotiating with the US over importing plasma for children.

The cost of importing the plasma is likely to run into millions of pounds.

Does anyone have any concerns about the idea?

Yes. Frances Hall, secretary of the Human BSE Foundation, warned there was no guarantee an American donor would not be infected with vCJD.

She said: "What is really needed is a blood test so each unit could be checked before it was used.

"I don't know how far off that is, unfortunately it does not exist yet. The long incubation period makes it difficult to detect."