Boris Johnson has stuck by his top aide Dominic Cummings, saying he "followed the instincts of every father and every parent" and "I do not mark him down for that".
The prime minister insisted his special adviser had "acted responsibly and legally and with integrity" after facing criticism for travelling from London to Durham during lockdown.
Mr Cummings was called into 10 Downing Street earlier on Sunday following reports he went into self-isolation at his family's farm 260 miles away with his child and wife, who had COVID-19 symptoms.
Several Tory MPs had called for him to be sacked for the journey that came while Britons were being told to "stay at home" to stop the virus spreading at the end of March.
But at the daily Downing Street coronavirus briefing, Mr Johnson said after "extensive face-to-face conversations" with Mr Cummings, he concluded the former Vote Leave boss "had no alternative" but to make the trip.
He added that Mr Cummings had been "travelling to find the right kind of childcare, at the moment when both he and his wife were about to be incapacitated by coronavirus".
Mr Johnson acknowledged the damage the allegations had the potential to do.