Prof Neil Ferguson has quit as a government adviser on coronavirus after admitting an “error of judgement”.
Prof Ferguson, whose advice to the prime minister led to the UK lockdown, said he regretted “undermining” the messages on social distancing.
The Telegraph reported that a woman he was said to be in a relationship with visited his home in lockdown.
Health Secretary Matt Hancock said it was “extraordinary” and that he “took the right decision to resign”.
He told Sky News that it was “just not possible” for Prof Ferguson to continue advising the government.
Mr Hancock said the social distancing rules “are there for everyone” and are “deadly serious”.
Scotland Yard said Prof Ferguson’s behaviour was “plainly disappointing” but officers “do not intend to take any further action”.
No 10 said the prime minister agreed with his decision to resign but Prof Ferguson was not told to do so and made the decision himself.
“Social distancing regulations are there for a very clear purpose,” the prime minister’s spokesman added.
Prof Ferguson’s modelling of the virus’s transmission suggested 250,000 people could die without drastic action.
This led Prime Minister Boris Johnson to announce on 23 March that he was imposing widespread curbs on daily life aimed at stopping the spread of the virus.
Under those measures people were told to go out as little as possible, with partners who live separately later being told they should “ideally” stay in their own homes.
In a statement, Prof Ferguson said: “I accept I made an error of judgement and took the wrong course of action.
“I have therefore stepped back from my involvement in Sage (Scientific Advisory Group for Emergencies).
“I acted in the belief that I was immune, having tested positive for coronavirus and completely isolated myself for almost two weeks after developing symptoms.
“I deeply regret any undermining of the clear messages around the continued need for social distancing.”
He also called the government advice on social distancing “unequivocal”, adding that it was there “to protect all of us”.
The Telegraph reported that Antonia Staats visited his home on at least two occasions during the lockdown.
Despite Prof Ferguson’s comments, it is currently unclear whether people who have recovered from the virus will be immune or able to catch it again.
Sir Robert Lechler, president of the Academy of Medical Sciences, said he did not think Prof Ferguson’s resignation would “have any material impact” on the work of Sage, which is advising the government on the pandemic.
He said that Prof Ferguson had made “an important contribution” but he was sure the group would “continue to provide valuable input”.
Security minister James Brokenshire said that “a range of experts” will continue to support ministers following Prof Ferguson’s resignation.