At least 5% of people in the UK have now developed COVID-19 antibodies, with the number rising to 17% in London, a study has found.
Health Secretary Matt Hancock gave the results of the study at the government's daily coronavirus update, as he announced plans for antibody certificates.
The figures are the first ones to be released by a government-commissioned study run by the Office for National Statistics, using 1,000 adults to track levels of immunity in the UK.
Participants had to give blood samples that are tested to check how many had developed COVID-19 antibodies.
Experts are still unsure what level of immunity recovering from the disease gives people and how long it might last.
But Prime Minister Boris Johnson has previously hailed an antibody test found by scientists at Porton Down to be 100% accurate as a "game-changer" for lifting the lockdown.
This, he said, is because someone with antibodies can be "safe and confident in the knowledge that you are most unlikely to get it again".
The test, made by Swiss pharmaceutical firm Roche, has been approved for use by Public Health England.
Mr Hancock confirmed it will be rolled out for free on the NHS from next week - going to health and social care workers first.
However, experts are still unsure what level of immunity recovering from the disease gives people - and how long the immunity might last.
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John Edmunds, professor of infectious disease modelling at the London School of Hygiene and Tropical Medicine, has warned studies of other coronaviruses suggest "potentially bad news" for hopes humans could develop long-term immunity.
He told a parliamentary committee on Tuesday: "We can also see from other coronaviruses, from ones that cause coughs and colds, that individuals again do seem to not have particularly long-term immunity to many of those viruses, allowing them to get reinfected later.
"Immunity may not last that long against this virus."
There have been more than 252,000 confirmed cases of COVID-19 in the UK, and at least 36,124 deaths.
Analysis: UK is still far short of herd immunity
Londoners are so much more likely to have had the infection because the capital was two weeks ahead of the rest of the country on the epidemic curve as we went into lockdown, writes our science correspondent Thomas Moore.
And since then spread of the virus has slowed dramatically.
These are results based on small samples of people across the country, so there will be a margin of error.
That’s why scientists are excited about the 10 million antibody tests that the government will roll out from next week.
They will give more reliable data with geographical and occupational breakdowns.
But the figures give us a useful guide to what has happened over the past few weeks.
And they show how far short we are of herd immunity.
Roughly 60% of the population would need to have had the virus and be immune to re-infection to stop the virus spreading.
It means a vaccine is our only realistic hope of returning to normal life.