Boris Johnson has been accused of risking the Government’s efforts to combat Covid-19 in order to defend his senior aide Dominic Cummings following allegations he breached lockdown restrictions.

The Prime Minister chose to front the daily Downing Street coronavirus briefing to publicly back Mr Cummings, saying he had “acted responsibly, legally and with integrity” by driving 260 miles to County Durham to isolate and that “any parent would frankly understand what he did”.

But Tory backbenchers tore into Mr Johnson over his handling of the row, while scientists claimed that the defence of Mr Cummings’ interpretation of the lockdown rules undermined efforts to curb the spread of Covid-19.

The storm over Mr Cummings’ actions overshadowed Mr Johnson’s latest signal that the lockdown is easing as the Prime Minister confirmed the phased reopening of England’s primary schools, starting on Friday.

Mr Cummings travelled to County Durham in March to self-isolate with his family while official guidelines warned against long-distance journeys, apparently because he feared that he and his wife would be left unable to care for their son.

Further reports also suggested he took a second trip to the North East in April, having already returned to London following his recovery from coronavirus – a disease which has seen more than 45,000 people in the UK die after contracting it, according to the latest available data.

Mr Cummings denied the fresh allegations, which were reported by the Observer and the Sunday Mirror, and Mr Johnson declared he would be standing by his most senior adviser.

Mr Johnson, leading the Government press conference for only the third time since being discharged from hospital on April 12, said he could “not mark down” Mr Cummings for the way he acted.

The Prime Minister said: “I have had extensive face-to-face conversations with Dominic Cummings and I have concluded that in travelling to find the right kind of childcare, at the moment when both he and his wife were about to be incapacitated by coronavirus – and when he had no alternative – I think he followed the instincts of every father and every parent.”

But Tory MPs criticised the Prime Minister’s actions, with former minister Paul Maynard saying he shared people’s “dismay” at the response.

“It is a classic case of ‘do as I say, not as I do’ – and it is not as if he was unfamiliar with guidance he himself helped draw up,” he said.

“It seems to me to be utterly indefensible and his position wholly untenable.”

Veteran Conservative Sir Roger Gale told the PA news agency: “I’m very disappointed, I think it was an opportunity to put this to bed and I fear that now the story is simply going to run and run.”

Somerton and Frome MP David Warburton said he was “unconvinced” by the PM’s defence of Mr Cummings.

Scientists advising the Government also strongly criticised the Prime Minister.

Social psychologist Professor Stephen Reicher, one of the scientists on the Scientific Pandemic Influenza Group on Behaviours (SPI-B) – a subgroup of the Scientific Advisory Group for Emergencies (Sage), which is advising ministers through the crisis – said Mr Johnson had “trashed” their advice.

Fellow SPI-B member Robert West, professor of health psychology at University College London (UCL), said the public must continue to socially distance despite the “confusion and misinformation” created by the Government.

Prof West told the PA news agency: “Dominic Cummings won’t suffer if we abandon it, the Prime Minister won’t suffer – it will be the people who we love who will suffer.

“Although we are fighting a rear guard action constantly against Government confusion and misinformation, we have to really keep hammering home this message.”

It came as:

– Mr Johnson confirmed at the briefing that the intention was for primary schools in England to open more widely on June 1, but acknowledged it “may not be possible” for all schools

– Police attended Mr Cummings’s London home on Sunday afternoon after it was “reported that a large crowd of people were outside the address”. Scotland Yard would not confirm who had called officers.

– An investigation was launched into a since-deleted tweet from the official UK Civil Service twitter account, posted 20 minutes after the news conference finished, stating: “Arrogant and offensive. Can you imagine having to work with these truth twisters?”

Over the weekend, Number 10 admitted Mr Cummings had driven from his London home to Durham in March after his wife started displaying Covid-19 symptoms, becoming fearful there would be no-one to look after his four-year-old child if he also took ill.

But according to reports, the former Vote Leave campaign co-ordinator made a second trip to Durham and was seen there on April 19, five days after being photographed on his return to Westminster.

A second witness told the papers they saw him a week earlier in Barnard Castle on Easter Sunday, a popular tourist location 30 miles from Durham.

Labour leader Sir Keir Starmer, said the decision to keep Mr Cummings on was “an insult to sacrifices made by the British people”.

“This was a test of the Prime Minister and he has failed it,” the Labour leader said.

Footage posted on social media showed Mr Cummings was heckled by a crowd of onlookers as he returned to his home in the capital, with those gathered calling him a “hypocrite” and shouting “resign”.

A woman filming Mr Cummings on her phone told him: “I’m a single parent. I’ve had no childcare since the beginning of this whole mess, not that I can afford to pay any child care.”

Another shouted: “Would you recommend Barnard Castle for a day out?”

Mr Cummings appeared not to respond.