Durham Police have formally been asked to "establish the facts" around Dominic Cummings' visit - including whether he broke the law.
Police Commissioner Steve White today wrote to the force's Chief Constable, Jo Farrell, asking her to see if any rules were broken when Mr Cummings visited County Durham at the end of March.
Retired teacher Robin Lees, 71, reported Mr Cummings to police yesterday after insisting he saw the top aide out in Barnard Castle - 30 miles from where he had stayed for two weeks - on April 12.
A witness also told the Sunday Mirror they saw Mr Cummings walking in County Durham on April 19, days after he returned to work in London from April 14. Ministers have flatly denied he made a second trip up north.
It will now be up to police to decide whether to launch an investigation.
A spokesman for Durham Constabulary said: "We can confirm that, over the last few days, Durham Constabulary has received further information and complaints from members of the public and we are reviewing and examining that information."
Ministers have vehemently denied Boris Johnson's top aide broke any lockdown rules or laws, despite driving 260 miles from London to stay near family while national orders were to "stay at home".
Education Secretary Gavin Williamson said today: "If he’s made it clear to the Prime Minister that he didn’t break the law, I absolutely believe that assurance.
"You wouldn’t expect someone to be not giving the absolute categoric truth to the Prime Minister.”
Boris Johnson last night said after "extensive face to face conversations with Dominic Cummings" he concluded the aide "followed the instincts of every father" - ensuring his son would be cared for if he fell sick.
The PM also said some allegations about his trip were "palpably false" but didn't say which ones.
It came as Boris Johnson summoned a bank holiday meeting of his Cabinet this lunchtime in an attempt to spell out how lockdown will ease from next Monday, and move on from the row.
The Prime Minister could spell out more details later today of how non-essential shops will start to reopen.
But he faces a mutiny from his own scientific advisors, bishops including the Bishop of Durham, and at least 20 Tory MPs who called for Mr Cummings to go.
Labour leader Keir Starmer called for a Cabinet Office inquiry and said he'd have sacked Mr Cummings. SNP Westminster Leader Ian Blackford MP added: "Boris Johnson must sack Dominic Cummings."
And shadow home secretary Nick Thomas-Symonds today wrote to Home Secretary Priti Patel, asking if someone taking a child cross-country in case they need care later would be fined by police.
“The confusion and misinformation caused by the government’s response to these revelations appears to make this work even more difficult,” he said.
Mr Thomas-Symonds also demanded any communications between Durham Police and the government since the story broke.
Mr White, Durham's Acting Police, Crime and Victims' Commissioner, said the row "has become a major issue of public interest and trust."
Lockdown laws say people can only leave their homes for a limited number of reasons, including care for a vulnerable person.
Mr White said in a statement: "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.
"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.
"It will be for the Chief Constable to determine the operational response to this request.
"I am confident that with the resources at its disposal, the Force can show proportionality and fairness in what has become a major issue of public interest and trust."
Today's statement comes after a bitter row between Durham Police and 10 Downing Street about the force's involvement in Mr Cummings' visit.
The Mirror and Guardian revealed on Friday that officers "made contact with the owners of that address" where Mr Cummings was staying.
Durham Police said in a first statement before the original story was published: "Officers explained to the family the guidelines around self-isolation and reiterated the appropriate advice around essential travel."
But on Saturday morning, No10 issued a statement saying: "At no stage was he or his family spoken to by the police about this matter, as is being reported."
That prompted Durham Police to hit back, reiterating that they had spoken to Mr Cummings' father at his own request on the phone and provided advice around security.
After that statement, ministers claimed the police contact was about security and not lockdown rules.
Transport Secretary Grant Shapps said Dominic Cummings’ father “contacted the police, not the other way round” and it was about the “unrelated matter” of security.
That had been disputed by sources inside Durham Police, according to the Times.
However, Durham Police then issued a fresh statement on Monday afternoon saying "our officer gave no specific advice on coronavirus to any members of the family".
The new statement on Monday said: "We can confirm that on April 1, an officer from Durham Constabulary spoke to the father of Dominic Cummings.
"Mr Cummings confirmed that his son, his son’s wife and child were present at the property. He told the officer that his son and son’s wife were displaying symptoms of coronavirus and were self-isolating in part of the property.
"We can further confirm that our officer gave no specific advice on coronavirus to any members of the family and that Durham Constabulary deemed that no further action was required in that regard.
"Our officer did, however, provide the family with advice on security issues."