The rule change – which comes into effect in England only at one minute past midnight on Saturday morning – applies only where an adult is living alone or with children aged under 18.
In these cases, the single-adult household can form a “support bubble” with one other household of any size.
Members of the bubble will effectively be able to behave as a single household during the lockdown, visiting one another as much as they like without any requirement to stay two metres apart, and staying overnight if they choose.
Announcing the move at the daily No 10 coronavirus briefing, Mr Johnson said it was aimed at those who have been alone during lockdown, but would not apply to people who are “shielding” because underlying health conditions make them particularly vulnerable.
“I know how difficult the past months have been for people cut off from their friends and family,” said the prime minister. “There are still too many people, particularly those who live by themselves, who are lonely and struggling with being unable to see friends and family.
“So, from this weekend, we will allow single adult households – adults living alone, or single parents with children under 18 – to form a support bubble with one other household.
“All those in a support bundle will be able to act as if they live in the same house, meaning they can spend time together inside each other’s homes and do not need to stay two metres apart. I will stress support bubbles must be exclusive – meaning you can’t switch the household you are in a bubble with or connect with multiple households.
“And if any member of the support bubble develops symptoms, all members of the bubble will need to follow the normal advice on household isolation.”
Mr Johnson added: “We’re making this change to support those who are particularly lonely as a result of lockdown measures. It’s a targeted intervention to limit the most harmful effects of the current social restrictions. It is emphatically not designed for people who don’t qualify to start meeting inside other people’s homes, because that remains against the law.
“Unfortunately, we cannot advise anyone who is shielding to form a support bubble at this stage, given their particular vulnerability to the virus. I know how hard it is for those of you who are shielding, and we will say more next week about the arrangements that will be in place for you beyond the end of June.”
The announcement came as the official UK death toll from the coronavirus pandemic hit 41,128, with 245 fatalities recorded over the most recent 24-hour period.
But Mr Johnson said it was possible to go ahead with the relaxation, along with moves to reopen non-essential retail, zoos and safari parks, because the government was confident that it was meeting its five tests on death and infection rates, provision of equipment to healthcare workers and keeping the all-important R rate of reproduction of the disease below one.
The chief medical officer, Chris Whitty, said the move was “a reasonable way to go at this point in time”, but added: “It’s not a risk-free one. There are no risk-free options.”
If the R rate spiked upwards, it might prove necessary to reverse this or other relaxations of lockdown rules, said Professor Whitty.
Downing Street said that the move would help widowed grandparents who have spent lockdown alone and will now be able to hug their grandchildren.
And single parents struggling to look after and home-school children on their own will be able to call on support from relatives or friends.
But it will also mean an end to the block – nicknamed the “sex ban” by some – on couples who do not live in the same property spending the night together.
Each household will only able to form a “bubble” with one other household, meaning that lone grandparents with a number of grown-up children will have to choose which of them to link up with.
And friends living in shared accommodation will have to decide which of them is able to invite a partner, friend or relative living alone into their bubble.
Separated or divorced parents who are already permitted to transfer children between their homes will each be able to link up with one other household – so long as they are not currently living with another adult.
Downing Street said that eligible households could form bubbles with addresses anywhere else in England, but it is expected that the majority taking advantage of the new freedom will involve properties reasonably close to one another.
The change will require amendments to coronavirus regulations, but No 10 said this can be completed by secondary legislation before Saturday without the need for a vote in parliament.
At the outset of lockdown, deputy chief medical officer Jenny Harries said that non-cohabiting couples would have to make a choice.
“If the two halves of a couple are currently in separate households, ideally they should stay in those households,” Harries said on 24 March.
“The alternative might be that, for quite a significant period going forwards, they should test the strength of their relationship and decide whether one wishes to be permanently resident in another household.”
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