A mass antibody test is at least a month away, a leading Government scientific adviser has said.
Professor Sir John Bell, from Oxford University, said the search was on for an antibody test that would prove effective, but those tested so far had failed.
Oxford is involved in scrutinising antibody tests that can tell people whether they have had the virus and can get back to normal life - one of the key routes out of the UK's current lockdown.
In a blog on the university website under the header "What next?", Prof Bell, who advises the Government on life sciences, said: "We will of course continue to look for a test that meets the criteria of an acceptable test."
He said the search was on for a test that was sensitive and specific enough that it could be taken at home.
"That should be achievable, and the Government will be working with suppliers both new and old to try and deliver this result so we can scale up antibody testing for the British public," he said.
"This will take at least a month."
Prof Bell said "large-scale testing" was a Government strategy "which will be crucial for getting us back to our normal lives in the coming months".
He explained that long-term protection to Covid-19 comes from antibodies - "small proteins produced by your immune system that attack the virus and neutralise it".
Prof Bell added: "These antibodies emerge during an infection and peak 28 days after the infection. They then remain for months afterwards continuing to provide protection, and the cells that produce them also are available to fight off the infection if it returns."
But he warned that creating home test kits for mass use is "not easy", adding there were currently 100 or more antibody tests that each had to go through validation.
Prof Bell said it had taken some time to get the right validation tools in place but said "the UK is now uniquely positioned to evaluate and find the optimal test for this disease".
He added: "We clearly want to avoid telling people they are immune when they are not, and we want all people who are immune to know accurately so they can get back to work.
"Sadly, the tests we have looked at to date have not performed well. We see many false negatives (tests where no antibody is detected despite the fact we know it is there) and we also see false positives.
"None of the tests we have validated would meet the criteria for a good test as agreed with the Medicines and Healthcare Products Regulatory Agency (MHRA). This is not a good result for test suppliers or for us."
He said the Spanish government had also returned test kits that were not working, "and the Germans who are developing their own sensitive kits believe they are three months away from getting these available and validated".
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