The isolation period after testing positive for Covid will be cut to just five full days from next Monday, to get staff back to work faster.
Sajid Javid, the health secretary, announced the cut in England, but said people would have to test negative on day six to escape isolation.
The scientific evidence is that two-thirds of Covid cases are “no longer infectious after the end of day five”, Mr Javid said.
Any curbs on people’s freedoms must be the “absolutely the last resort” and should not remain in place “a day longer than absolutely necessary”.
The statement kicked off further clashes over the prime minister’s confession that he did attend the No 10 garden party on May last year, after days of obfuscation.
He is “not fit to lick the boots of NHS workers”, Labour’s shadow health spokesperson, Wes Streeting, alleged.
At present, people are required to isolate for seven days from the point at which they have symptoms or test positive – with release after two negative lateral flow test results on days six and seven.
Business leaders had urged the government to follow the example of the US and ease the rules, warning of the impact of rising staff absences – and of customers from pubs and restaurants.
On Monday, the UK Health Security Agency was left red-faced after admitting it was wrong to claim that the US five-day isolation period kicked in later than the British seven-day rule.
It had claimed isolation only started after a positive test, not when symptoms were first experienced – but backtracked after being corrected by the US Centers for Disease Control.
Last week, Mr Johnson said of five-day isolation: “The risk is you would increase the numbers of people going back into the workplace who are infectious by a factor of three,” but he then switched tack.
However, the UKHSA has said its error had not changed its conclusion that allowing people to leave isolation earlier will increase the risk of them spreading Covid.
Mr Javid told MPs: “After reviewing all of the evidence, we’ve made the decision to reduce the minimum self-isolation period to five full days in England.
“From Monday, people can test twice before they go – leaving isolation at the start of day six.
“These two tests are critical to these balanced and proportionate plans, and I’d urge everyone to take advantage of the capacity we have built up in tests so we can restore the freedoms to this country while we’re keeping everyone safe.”
Labour backed the move, but said shortages of lateral flow tests still needed to be “sorted out”.
“Over Christmas, NHS staff and other key workers were unable to access tests because the government hadn’t noticed that the deliveries had shut up shop for Christmas,” Mr Streeting alleged.