Eighteen British troops wounded in Iraq and Afghanistan are undergoing medical checks after it emerged they received blood which was not properly tested.
The affected troops were given the transfusions while on the front line
The emergency transfusions came from US personnel, who have subsequently been found not to have hepatitis or HIV.
Six UK civilian security contractors may also have received contaminated blood provided by the US military, the Health Protection Agency said.
The MoD said the infection risk was low and the transfusions had been vital.
Without the blood the troops may have died, it said.
However, the US donors did not undergo full "valid retrospective tests" to screen them for diseases," the MoD said.
The British civilians are understood to have been private security contractors working for the American military.
They were given blood sourced from the US military because they were seriously wounded and their lives were at risk.
The Health Protection Agency said the GPs of all six had been informed.
As well as HIV, the British personnel are being tested for hepatitis B and C, HTLV - a virus similar to HIV, Chagas - a tropical parasitic disease which occurs in the Americas - and the sexually-transmitted infection syphilis.
The 18 troops had been serving in the countries since 2001.
The MoD said it was contacting every one, but advised any member of the forces concerned about a blood transfusion to contact their own doctor or medical officer.
It admitted that the blood was not subject to retrospective tests, which are used to screen donors for diseases after an emergency transfusion.
Douglas Young, the chairman of the British Armed Forces Federation, told BBC Five Live that such checks were vital after emergency transfusions.
"If the necessary testing couldn't take place before the blood was given at all, then clearly there should have been what is called retrospective testing, where the donors or the blood batches are checked to ensure there are no issues involved," he said.
"Because in some circumstances, if there is a problem, it's as well to know that quickly, because some kinds of treatment may be given that isn't possible a year or two years later."
Defence Minister Derek Twigg said that although the risk of troops being infected was low, the MoD was not taking the situation lightly.
"These 18 service personnel would almost certainly have died without receiving an emergency blood transfusion at the front line," he said.
He added: "We are working with the appropriate health authorities to do all that we can to test and reassure the people involved.
"We continue to do all that we can to support them and their families through this uncertain time."
Gordon Brown's spokesman said: "This is a very serious matter and the Ministry of Defence are taking it very seriously."
A US Department of Defense spokesman said the US military did not have the means or time to properly screen donors on the battlefield during combat operations.
He added: "The UK and US co-operate closely in developing world-class, life-saving combat medical care and we'll continue to work closely together to monitor this situation as the recipients' test results are completed and further information becomes available."
He added that the American donors had since been screened for HIV and hepatitis, and proved negative, meaning that "the chance of infection in British troops is very low".
The MoD said it was routine procedure in emergencies for British military personnel to be treated at the nearest medical facility, whether it was run by American, coalition or UK forces.
A spokeswoman said the test results were expected within the next three weeks.