The British soldiers, who were seriously wounded, are now undergoing tests after it emerged that the Americans did not follow their own procedures and test the donors after they gave the blood.

The Health Protection Agency said the GPs of all six affected civilians had been informed. But it admitted that because of the holiday period, some of the GPs may not yet have informed their patients. Those affected are being offered testing and counselling.

An HPA spokeswoman said: "The Health Protection Agency is aware that a small number of British individuals may have received a transfusion of unscreened blood as a result of life-threatening injuries received in Afghanistan or Iraq.

"The agency has advised the Ministry of Defence on the level of risk to those who may have been potentially exposed to infections as appropriate and is assisting with contacting some of the affected people."

Haemophiliacs who were infected with HIV and hepatitis C after receiving contaminated blood in the late 1970s and 1980s described the blunder as an "absolute disgrace".

Robert Mackie said it was "outrageous" that servicemen and women who risked their lives for their country had been put in this position. He said: "Attitudes have not changed, it is as simple as that. Lessons have not been learnt. It's an absolute disgrace, to say the least."

Defence Minister Derek Twigg said the 18 soldiers would "almost certainly" have died if they had not received an emergency blood transfusion at the front line. While the risk of infection is "low" the matter is being taken "extremely seriously", he said.

The US Department of Defence said the American donors who provided the blood had now all tested negative for hepatitis and HIV. But the British soldiers are still facing an agonising wait to learn the results of their own tests for blood infections.

Prime Minister Gordon Brown's spokesman said: "Of course this is a very serious matter and the Ministry of Defence are taking it very seriously."