Couples who live in separate homes should "test the strength" of their relationship and move in together during the coronavirus outbreak, a medical expert said on Tuesday.
Dr Jenny Harries, the deputy chief medical officer, urged people to remain within one household to stop the spread of Covid-19, stressing that the rules applied to live-apart boyfriends and girlfriends.
Joking that she did not want to start a new career in "relationship counselling", she said: "The principle is that we want people to stay in their household units primarily. If you have an infection, you are very close to your family members so the risk of exposure is very similar.
"If you've two individuals, two halves of a couple, currently in separate households, ideally they should stay in those households.
"The alternative might be that, for quite a significant period going forward, they should test the strength of their relationship and decide if one wants to be a permanent resident of another household.
"The issue here is what we do not want is people switching in and out of households. It defeats the purpose in the reduction of social interaction. Otherwise we will not all be working towards achieving the outcome."
Matt Hancock, the Health Secretary, who joined Dr Harries for Downing Street's first virtual press conference, with journalists asking questions via conference call, added: "Make your choice and stick with it."
The clarification came amid confusion over whether couples living apart were still allowed to meet up under the Government's new lockdown rules, which prohibit meetings of more than two people who do not live together.
People who flout the lockdown will be fined £30 from Thursday, with Downing Street threatening "significantly" higher penalties for those who continue to disobey the new measures.
On Tuesday, the Prime Minister's spokesman appeared to indicate that lovers should not see each other if they live in separate homes, in order "to save lives".
He later said: "We are working at pace and, as people raise these issues with us, we will get clarity for them as soon as possible".
The spokesman declined to comment on suggestions that punishment fines could reach £1,000, but warned: "We will keep this under review, and can increase it significantly if necessary in order to ensure public compliance."
Asked how the lockdown would be enforced, he said: "As is usual, the police will use their own discretion in their use of the powers and will do whatever is appropriate."