A Northern Ireland doctor has said he is "cautiously optimistic" after a test to find out whether people have been infected with coronavirus in the past was approved by health officials.
Public Health England (PHE) said the antibody test, developed by Swiss pharmaceutical giant Roche, was a "very positive development" after experts at its Porton Down facility gave it the green light.
The test, which Prime Minister Boris Johnson has previously called a "game-changer", picks up 100% of cases where somebody has had coronavirus in the past.
Experts believe those who have had Covid-19 develop a degree of immunity, meaning the test could prove a useful tool for helping to ease lockdown restrictions.
Number 10 said the new antibody test would "certainly" be available on the NHS but commercial discussions with Roche are ongoing.
The Prime Minister's official spokesman said the idea of an "immunity certificate" was still under consideration if science showed that people developed immunity to Covid-19.
Professor John Newton, national coordinator of the UK Coronavirus Testing Programme, said although it was still unclear to what extent the presence of antibodies indicated immunity, the test was a "very positive development".
He added: "Last week scientific experts at PHE Porton Down carried out an independent evaluation of the new Roche Sars-CoV-2 serology assay in record time, concluding that it is a highly specific assay with specificity of 100%.
"This is a very positive development because such a highly specific antibody test is a very reliable marker of past infection.
"This in turn may indicate some immunity to future infection, although the extent to which the presence of antibodies indicates immunity remains unclear."
Dr Alan Stout, from the British Medical Association NI, said he was "cautiously optimistic" about the breakthrough.
Speaking on Talkback, he said there were still many unanswered questions.
Dr Stout said it was important to remember there was no guarantee people cannot get infected again and said "complacency" must not creep in.
Roche said it could supply hundreds of thousands of the tests each week. The tests run on fully-automated equipment already widely installed by Roche at NHS sites across the UK.
The pharmaceutical firm said it would prioritise tests for distribution via the NHS before looking at how they may be sold to individuals.
Professor Sir John Bell, regius professor of medicine at Oxford University, said the development of the antibody test was "a good result".
He told Radio 4's Today programme: "It's a step in the right direction. In the evolution of these antibody tests, to get one that works really well is a major step forward."
Sir John said antibodies "stick around probably for a year or two", adding that the Roche test was the "best approved test available on the market now."
But he said it was currently unclear whether having Covid-19 gave immunity against all future infection with the virus, adding improvements could still be made to antibody tests.
Health minister Edward Argar said the Government intends to roll out the new test to frontline workers first.