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”.
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.