The UK has placed antibody tests – which check if someone has had Covid-19 – at the centre of an eventual “back-to-work” plan to restart normal life.
But experts said they may not prove if someone is protected from reinfection.
The UK’s testing co-ordinator has also warned people not to buy private tests.
Already, the government has paid for three-and-a-half million antibody tests, but has not yet found one that is reliable enough to use – and stresses that it will not approve the use of any test until it can be sure its findings can be fully depended on.
Professor John Newton said the public should not purchase unapproved antibody tests until a working test is approved.
“We are breaking new ground with this work every day and I am confident this major research effort will make a breakthrough,” he said of efforts to develop a valid serology test, which measures levels of antibodies in blood plasma.
“Until then, please don’t buy or take any unproven tests. They may not be reliable for your intended use; they may give a false reading and put you, your family or others at risk.”
He added: “As soon as we have found a test that works for this purpose, we will be in a position to roll them out across the country as a back-to-work test.”