No10 has admitted Boris Johnson 'commuted' between Downing Street and Chequers in March 2020.
The Prime Minister was based at the grace and favour mansion in Buckinghamshire for more than 10 days after he called for an end to non-essential travel on March 16.
However, Downing Street has denied the Prime Minister broke any lockdown regulations.
A spokesman said Mr Johnson and his then fiancée Carrie Symonds were based at Chequers from March 16 to March 27 because she was heavily pregnant and advised to minimise social contact.
The UK's first lockdown was announced on March 23 and people were legally prohibited from visiting second homes on March 26.
Tortoise Media revealed Mr Johnson's Chequers stay after quizzing Downing Street for more than a year about the affair.
A No 10 spokesman said: “This claim that the Prime Minister did not comply with lockdown regulations and guidance is entirely inaccurate”.
“At that time, Mrs Johnson was heavily pregnant and had been placed in a vulnerable category and advised to minimise social contact.
“In line with clinical guidance and to minimise the risk to her they were based at Chequers during this period, with the Prime Minister commuting to Downing Street to work.”
Sources told Tortoise Media that staff members at Chequers caught Covid during Mr Johnson's stay in March 2020.
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The latest revelation comes after Mr Johnson admitted to being at a 'bring your own booze' party in the garden of No10 on May 20 2020.
Gatherings were banned in England at the time under Covid restrictions.
Apologising for his actions, the Prime Minister told MPs that he “went outside for 25 mins" to thank staff but admitted "with hindsight I should have sent them back indoors".
But he still tried to wriggle out of censure by saying: "I believed implicitly that this was a work event."
Labour leader Keir Starmer branded his defence "ridiculous" and demanded to know if the PM would resign.
And it appeared the PM had not won over his critics, with Tory peer Ruth Davidson saying "I'm not convinced today drew a line under anything".
Campaigners for bereaved families said the PM's "lies have finally caught up with him" - and he must resign.
A leaked email from the PM's principal private secretary, published by ITV on Monday, showed No 10 staff were invited to "make the most of the lovely weather " with a garden party on May 20 2020.
More than 100 No10 staff were invited to the drinks party.
Witnesses said about 40 staff attended, including Boris Johnson and his wife Carrie - and there were long tables laden with drink, crisps, sausage rolls and other picnic food.
