Dominic Cummings will make a rare public statement on Monday afternoon as the prime minister faces a revolt from within his own party.
MPs from across the political spectrum are lining up to condemn Boris Johnson and his chief adviser.
A storm of protest continues to rage not only over revelations the prime minister’s most senior and most trusted aide broke the government’s own lockdown guidelines, but also due to Mr Johnson’s defence of Mr Cummings on Sunday.
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.
As a government advisor, a public statement and questions are extremely rare.
ITV News Political Correspondent Paul Brand said he "cannot recall an advisor ever being publicly questioned in this manner before.
"Downing Street going to extraordinary lengths to try and keep hold of the PM's top aide and put the story to bed."
Meanwhile, the Prime Minister is chairing a Cabinet meeting where ministers are expected to discuss the easing of restrictions for certain sectors of the economy, including the reopening of some non-essential shops.
On Sunday, Mr Johnson fronted Downing Street’s coronavirus briefing to back Mr Cummings, saying he had “acted responsibly, legally and with integrity” and that “any parent would frankly understand what he did”.
However Durham Police has been asked to "establish the facts concerning any potential breach of the law" surrounding Mr Cummings’ visit to the county.
In a statement, the force’s acting police, crime and victims’ commissioner Steve White said: "I am confident that thus far, Durham police has responded proportionately and appropriately to the issues raised concerning Mr Cummings and his visit to the County at the end of March."
"It is clear however that there is a plethora of additional information circulating in the public domain which deserves appropriate examination."
He added: "I have today written to the Chief Constable, asking her to establish the facts concerning any potential breach of the law or regulations in this matter at any juncture."
"It is vital that the force can show it has the interests of the people of County Durham and Darlington at its heart, so that the model of policing by consent, independent of government but answerable to the law, is maintained."
Mr Cummings' actions have sparked fury among some MPs, and led to warnings that he has "undermined" efforts to fight coronavirus.
Professor Stephen Reicher, a member of the Government's advisory group on behavioural science, told ITV's Good Morning Britain: "If you look at the research it shows the reason why people observed lockdown was not for themselves, it wasn't because they were personally at risk, they did it for the community, they did it because of a sense of 'we're all in this together'.
"If you give the impression there's one rule for them and one rule for us you fatally undermine that sense of 'we're all in this together' and you undermine adherence to the forms of behaviour which have got us through this crisis."
Gloucestershire's independent police and crime commissioner Martin Surl said Mr Cummings' actions made a "mockery" of police enforcement earlier in the lockdown.
He told BBC Radio 4's Today programme: "I think it makes it much harder for the police going forward - this will be quoted back at them time and time again when they try to enforce the new rules.
"But I think more importantly it makes something of a mockery of the police action going back when the message was very, very clear: stay at home."
Mr Cummings' alleged actions have also sparked dissent inside Tory party ranks.
Somerton and Frome MP David Warburton told ITV News "sacrifices" were being made across the country in a bid to stop coronavirus spreading and the fact that there are rules in place meant "there ought not to be room for any sort of interpretation or following of one's instincts and allowing that to override the guidance we've been given".
Mr Warbuton added his father had passed away several weeks ago, but neither he nor anyone else was "able to be there when he died" due to lockdown rules.
MP Peter Bone told ITV News: "The vast, vast majority of Conservative MPs think Dominic Cummings should go".
"What I think is unforgivable is that Dominic Cummings didn't resign," he said.
Mr Bone said: "And then he put the prime minister in a situation where it was either sack him or keep him and, in the past, an adviser in that situation would have no hesitation but to resign."
Mr Bone added that "when advisers become the story, they go".
"Behind the scenes, the vast, vast majority of Conservative MPs think Dominic Cummings should go," he said.
Drawing attention to "the moral hazard of Cummingsgate", Tory MP George Freeman retweeted an article from The Spectator which said Mr Johnson’s judgement was "now the issue".
In another tweet, Mr Freeman appeared to bemoan what was missing from the responses of the PM and his main adviser, saying: "Today we needed: some humility; a clear acknowledgement that people would be rightly angry if they sensed double standards; a sincere thank you to the millions of people (including fathers) who have made sacrifices Dominic Cummings didn’t; and a public apology from him."
Despite the anger in Tory ranks, Cabinet members have thrown their support behind Mr Johnson and Mr Cummings.
Gavin Williamson said Mr Cummings had given a "very clear assurance that no rules had been broken, no laws have been broken and the prime minister has very clearly taken that assurance and is very much supporting Dominic Cummings.
"If that's good enough for the prime minister, it's certainly good enough for me," England's education secretary told ITV News.
He continued: "The prime minister has asked for assurances about his actions, he's got those assurances, he's been given them by Dominic Cummings - I think the prime minister can ask no more.
"If someone hasn't broken the rules and hasn't broken the law it's right that the prime minister has the integrity to stand by them."
Labour leader Sir Keir Starmer tweeted Mr Johnson had failed a test of leadership, saying his decision to take no action against Mr Cummings was "an insult to sacrifices made by the British people".
While Steve Reed, the shadow communities and local government minister told ITV News that Mr Cummings' actions had "sown huge confusion" and called for an inquiry into the facts.
"Millions of British people have made extraordinary sacrifices over the last few weeks, people have not gone to loved ones' funerals, people have not been able to say goodbye to dying parents even," Mr Reed said.
"People have the right to expect that the guidelines they follow to protect lives are also being followed by the government and the government's senior advisors.
"The prime minister needs to move swiftly now to re-establish the trust of the British people in their guidelines if they expect them to follow them, because what this whole episode over the weekend has done is sown huge confusion - people are emailing me to ask whether they do or don't need to follow the guidance.
"That has to be sorted because lives are at risk...
"The key thing is we need to restore the government's moral authority to issue guidance that everyone follows because they know the government is following it themselves.
"That doesn't appear to be the case with Dominic Cummings' trip across the country, so what needs to happen now is the prime minister should order an inquiry by the Cabinet secretary to establish the actual facts and then in the full light of that knowledge a decision can be taken about whether Mr Cummings did or didn't breach the rules and what needs to happen as a consequence."
Scottish First Minister and SNP leader Nicola Sturgeon said Mr Cummings should either resign or be sacked, pointing out she had had to accept the resignation of Scotland’s chief medical officer adviser Catherine Calderwood last month for her own lockdown breach.
"I know it is tough to lose a trusted adviser at the height of crisis, but when it’s a choice of that or integrity of vital public health advice, the latter must come first," Ms Sturgeon tweeted.
"That’s the judgment I and, to her credit, Catherine Calderwood reached. PM and Cummings should do likewise."
Acting Liberal Democrat leader Ed Davey tweeted “Cummings must go”, saying the public would be “confused and angry” that he is still in his position.
And LibDem peer Lord Rennard called for Mr Cummings’ sacking and for his full disclosure, citing Mr Cummings’ handling of last year’s sacking of Treasury media adviser Sonia Khan as a precedent.
Coronavirus: Everything you need to know