But the British authorities and the test's manufacturer last night dismissed fears that this could put blood recipients at risk and claimed the test was still at the top of the safety league for detecting the Aids virus.
The study carried out by the State Serum Institute in Copenhagen, Denmark, claims the test kit now being used in the United Kingdom performed poorly at detecting a type of the virus, known as HIV-2.
Most tests for HIV will detect the virus about 30 days after infection, which leaves a small "window" of risk in the first few weeks when the virus cannot be spotted.
The new test detects HIV-1, the most common form of the virus, at an early stage. But in the Danish study, it failed to detect HIV-2 until six weeks later than other kits. Blood service insiders said the problem meant infected blood could slip through the testing procedure - which checks all donations because blood cannot be heat-treated to kill HIV - putting patients at risk.
Dr Claus Bohn Christiansen, who carried out the study published in the international journal, Vox Sanguinis, said: "I am very surprised that the British National Blood Authority is using this kit because it isn't very effective at detecting the virus.
"It raises the chance that someone infected with Aids could give blood and that it would not show until the virus had been in their body for more than two months."
But Sue Cunningham, for the NBA in England and Wales, said the findings were based on one particular case. She added: "There is a UK committee that looks at all the different tests that become available and this is one that has been accepted."
"It is UK policy to accept the most up-to-date tests that have been evaluated by the manufacturers. This one is very good at picking up HIV-1. We don't get many cases of HIV-2."Reuse content