Pressure is mounting on the PM to act over his senior aide’s lockdown trip, as the cabinet is to meet later to discuss plans to ease restrictions.
Senior Church of England bishops and a scientist advising ministers on the pandemic have strongly criticised the government’s handling of the row.
Acting Lib Dem leader Sir Ed Davey said it undermined the PM’s authority and Labour has called for a inquiry.
Boris Johnson has defended Dominic Cummings, who travelled to Durham.
Speaking at Sunday’s Downing Street briefing, the prime minister said he believed Mr Cummings had “no alternative” but to make the journey from London at the end of March for childcare “when both he and his wife were about to be incapacitated by coronavirus”.
At the press conference, Mr Johnson also confirmed the phased reopening of England’s primary schools will begin on 1 June.
The prime minister is this week expected to set out details of plans to ease restrictions.
Plans to be outlined by the government will reportedly include information about the reopening of some non-essential shops in June.
However, BBC political correspondent Iain Watson said Mr Johnson is finding it difficult to shift the political focus away from his key adviser.
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- What do we know about Cummings’ lockdown trip?
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- Coronavirus Newscast: The Dominic Cummings row
Speaking on BBC Breakfast, Sir Ed said that the row over Mr Cummings was “preventing the government from getting on and doing its job, and doing it better”.
He said that the prime minister should sack Mr Cummings “so the government has more credibility in what it says about public health”.
Sir Ed added: “The instruction the prime minister gave us all to stay at home has been breached by his top adviser and that’s what you can’t get away from in this story, its pretty simple.
“I hope the prime minister will come to his senses, recapture his judgement and reinstall authority on this crisis by acting.”
Education Secretary Gavin Williamson told the programme that Mr Cummings “has made it absolutely clear to the prime minister that he broke no rules and he broke no laws”, and insisted that he should not resign.