Boris Johnson has brushed aside a growing revolt of almost 100 of his own MPs and defied ongoing calls for Dominic Cummings to be sacked, despite a police investigation that failed to exonerate him for a potential breach of lockdown. The prime minister was again besieged by questions about his chief adviser as the crisis overshadowed his decision to cautiously ease the lockdown in England, which will require millions of citizens to abide by the letter and spirit of new guidelines. Johnson said the matter was closed after Durham police concluded Cummings may have broken lockdown rules but decided they would take no action against him. The prime minister also blocked journalists from asking his top scientific advisers, Prof Chris Whitty and Patrick Vallance, about their expert opinion on the matter. Cummings’ actions remained in the spotlight for a sixth day, however, after the police concluded that his family trip to Barnard Castle, a Teeside beauty spot, may have been in breach of “stay at home” guidance and would have led officers to tell him to turn around had he been stopped. A police examination of the facts surrounding Cummings’ time in the north-east, 260 miles from his London home, has concluded that no further action will be taken. The force said it would not issue fines retrospectively. Cummings has denied any wrongdoing following a joint investigation by the Guardian and the Daily Mirror, and has received Johnson’s full backing since the details were published last weekend. Deep discontent among Tory MPs has grown over the course of the week, however, in response to public anger. Ninety-nine have either called for Cummings to be sacked, demanded an apology or otherwise criticised his conduct. The figures suggest nearing half of the party’s backbenchers are unhappy with Cummings’ actions, while the vast majority of the “payroll vote” – government ministers, whips, parliamentary private secretaries and those in party positions – are supportive of Johnson’s position or silent on it. Tory MPs to break their silence most recently include Karen Bradley, a former culture secretary under Theresa May, who said she was not convinced by his explanation. “I can well understand that any parent wants to protect their child, and have enormous sympathy with acting to do that. But so much of the account goes beyond that simple human instinct and suggests that there were breaches of the rules to avoid some of the hardships that many have endured,” she said. “Mr Cummings is, in my view, causing significant damage to the work that the government are doing to get us through the pandemic. I hope that he will recognise this and consider his position.” Two former work and pensions secretaries also weighed in. Esther McVey, like Cummings an ardent Brexiter, said she would have resigned her position had she been in the same situation, and Stephen Crabb said Cummings was “wrong to arrogantly brush away the allegations when they first appeared”. “He just made matters worse. People have got every right to ask what on earth he was doing in Durham and Barnard Castle during lockdown. I would have preferred to see the cabinet secretary do a proper inquiry into the allegations and for Cummings to step aside to allow that to happen,” he said. Johnson avoided any mention of his chief adviser’s name during the daily No 10 press briefing. He also twice prevented questions from journalists who wanted to know whether Whitty, the chief medical officer for England, and Vallance, the chief scientific adviser, believed Cummings had stuck to the rules. He interjected to say he wanted to protect the two scientists “from what I think would be an unfair and unnecessary attempt to ask any political questions”. On a third time of asking, Whitty and Vallance said they did not want to get involved in politics. Whitty said: “The desire to not get pulled into politics is far stronger on the part of Sir Patrick and me than it is in the prime minister.” Vallance added: “I’m a civil servant, I’m politically neutral, I don’t want to get involved in politics at all.” Their silence on the matter will fuel suspicions that they are not willing to back up the prime minister’s assertions that he believes Cummings did not break the lockdown rules. There have been questions over the absence of the UK’s two most senior experts from the daily press conference since news broke of Cummings’ trip from London to Durham while his wife was showing possible coronavirus symptoms. Whitty and Vallance were due to appear at Monday’s press conference but were replaced at the last minute by Prof Pauline Doyle, an expert from Public Health England. The deputy chief medical officer, Jenny Harries, previously answered questions about Cummings actions. “If you’re symptomatic, you stay at home, take yourself out of society as quickly as you can and stay there, unless there’s extreme risk to life,” she said. Cummings said in his statement on Monday that he suspected he and his wife may have contracted coronavirus, but that neither of them had classic symptoms before they travelled to Durham to seek childcare if needed. Labour also kept the pressure on Johnson. Keir Starmer, the party leader, said it was “extraordinary that the prime minister stopped the scientists being able to answer a question put to them by journalists”. Answering questions on a virtual Q&A with voters, Starmer said this reflected badly on Johnson: “The government has been saying for weeks on end, this is about transparency, it’s about answering whatever questions you’ve got. And then on the question people want to hear an answer on, he stops them answering. It’s the wrong thing to do. “I don’t think he’s going to have impressed anybody with that tonight. I don’t think he’s impressed anybody over the past week.” Cummings’ actions and No 10’s defence of them had stirred something “personal and emotional” in the public, Starmer said. “Lots of people are beating themselves up because they’re not doing something for somebody they love, and that’s why when they saw Cummings apparently saying ‘that’s your rule, but I don’t necessarily have to follow it,’ it really struck not a party political chord but something deep in everybody.” Nicola Sturgeon, the first minister of Scotland and SNP leader, said she wanted to move on from talking about Cummings, but also made clear she thought he “should no longer be in post”. “That is a matter for the prime minister,” she said. “But I think if the prime minister is putting the integrity and trust in the vital public health message that he is seeking to communicate and all of us are seeking to communicate first then he will take action that means Dominic Cummings does not continue to distract from that.” A No 10 spokesperson said: “The police have made clear they are taking no action against Mr Cummings over his self-isolation and that going to Durham did not breach the regulations. The prime minister has said he believes Mr Cummings behaved reasonably and legally given all the circumstances and he regards this issue as closed.”