Health and care staff will be the first to receive antibody tests to check if a person has had coronavirus, after the government agreed a deal with a large pharmaceutical company.
The tests will be available on the NHS “for people who need them”, No 10 said.
At the moment, the only testing available to all adults and children aged over five are swab tests to check if someone currently has Covid-19.
The deal follows talks between the government and Swiss firm Roche.
It comes as the government announced on Thursday a further 338 people had died after testing positive for the virus.
The prime minister’s official spokesman said: “The tests will be free for people who need them, as you would expect.
“NHS and care workers will be prioritised for the tests.”
The Health Secretary Matt Hancock is expected to give more details this evening.
It comes after NHS England’s medical director Prof Stephen Powis cautioned people against using antibody tests which are being sold by some retailers.
On Wednesday, Superdrug became the latest business – and first High Street retailer – to offer the antibody test. The kit costs £69 and buyers need to take a blood sample at home, which is sent off to a lab for testing.
The coronavirus tests being used by the NHS already involve taking a swab up the nose or from the back of the throat. These tests tell you if you currently have Covid-19.
The antibody test is a blood test that looks for antibodies in the blood to see whether a person has had the virus.
Antibodies are made by our immune system as it learns to fight an infection. However, having antibodies does not automatically mean you cannot get sick or harbour the virus and pass it on to others, says BBC health correspondent James Gallagher.
The World Health Organization says there is no evidence people who have recovered from Covid-19 and have antibodies are protected from being infected again.
Last week, Public Health England approved an antibody test developed by Roche, calling it a “very positive development”.
The government previously spent a reported £16m buying antibody tests which later proved to be ineffective.
Public Health England said experts at the government’s Porton Down facility had evaluated the Roche test.
Roche found that if someone had been infected, it gave the correct result 100% of the time.
If someone had not caught coronavirus then it gave the correct result more than 99.8% of the time.
It means fewer than two in 1,000 healthy people would be incorrectly told they had previously caught the coronavirus.
Health minister Edward Argar previously said the tests would mainly be used on those in the NHS and social care settings to begin with.