Video report by ITV News Reporter Ben Chapman
Mr Cummings says he made the journey for childcare purposes. Here is the timeline of events around his trip.
As the coronavirus crisis escalates, the UK is placed into lockdown with strict limitations on travel.
The government guidelines state: "You should not be visiting family members who do not live in your home."
Mr Johnson and Health Secretary Matt Hancock test positive for coronavirus, while chief medical officer Chris Whitty says he has symptoms of the disease and is self-isolating.
Transport Secretary Grant Shapps told the BBC he believes Mr Cummings was last seen in Downing Street on the same day and speculates he may have travelled either on the 27th or 28th.
Downing Street confirms Mr Cummings is suffering from coronavirus symptoms and is self-isolating.
Durham police are "made aware of reports that an individual had travelled from London to Durham and was present at an address in the city".
The force says officers "made contact with the owners of that address who confirmed that the individual in question was present and was self-isolating in part of the house".
"In line with national policing guidance, officers explained to the family the arrangements around self-isolation guidelines and reiterated the appropriate advice around essential travel."
An unnamed neighbour tells the Daily Mirror and the Guardian that Mr Cummings was seen in his parents’ garden.
"I got the shock of my life as I looked over to the gates and saw him," they said.
The Guardian approaches Downing Street about the story, only to be told by a spokesperson: "It will be a no comment on that one."
March 30 to April 6:
The period for which Mr Cummings’ wife Mary Wakefield describes the family’s battle with coronavirus, in the April 25 issue of the Spectator.
She makes no mention of the trip to Durham and describes the challenges of caring for their son while suffering the symptoms of Covid-19.
"This might be my only really useful advice for other double-Covid parents or single mothers with pre-schoolers: get out the doctor’s kit and make it your child’s job to take your temperature.
"Any game that involves lying down is a good game."
Number 10 is again contacted for comment regarding Mr Cummings’ trip by the Guardian. Instead of defending the journey, officials decline to comment.
Mr Cummings and his family are apparently spotted out walking 30 miles away from Durham in Barnard Castle, according to Robin Lees, 70, a retired chemistry teacher, whose claim is reported by the Observer and Sunday Mirror on May 24.
Mr Cummings returns to work for the first time since news he was suffering from coronavirus emerged.
Questions are raised about his adherence to social distancing advice as he is photographed walking in Downing Street with fellow aide Cleo Watson.
An unnamed witness apparently sees Mr Cummings out walking with his wife in Durham, recognising him by his trademark beanie hat and overhearing him remarking that the bluebells are "lovely". The claim is reported by the Observer and Sunday Mirror on May 24.
The government lifts the restriction on how far people can drive to reach the countryside and take exercise, but visits and overnight stays to second homes remain prohibited.
News breaks in the Mirror and the Guardian of Mr Cummings’ trip to Durham.
While there is no comment from Downing Street, close friends of Mr Cummings say: "He isn’t remotely bothered by this story, it’s more fake news from the Guardian. There is zero chance of him resigning."
Downing Street appears to be standing by the PM’s chief aide, saying in a statement: "Owing to his wife being infected with suspected coronavirus and the high likelihood that he would himself become unwell, it was essential for Dominic Cummings to ensure his young child could be properly cared for.
"At no stage was he or his family spoken to by the police about this matter, as is being reported."
Speaking to reporters outside his home, Mr Cummings says: "I behaved reasonably and legally."
When a reporter suggests his actions did not look good, he replies: "Who cares about good looks? It’s a question of doing the right thing. It’s not about what you guys think."
Later at the daily Downing Street briefing, Mr Shapps says Mr Cummings has the PM’s "full support" and that Mr Johnson "knew that he was unwell and that he was in lockdown".
Mr Shapps says it had always been permissible for families to travel to be closer to relatives as long as they "go to that location and stay in that location".
Mr Johnson also pledged his "full support" on Saturday to his under-fire chief adviser.
Meanwhile, the deputy chief medical officer for England, Jenny Harries, says travelling during lockdown was permissible if “there was an extreme risk to life”, with a “safeguarding clause” attached to all advice to prevent vulnerable people being stuck at home with no support.
In a new statement released later in the evening, Durham police say officers were made aware on March 31 that Mr Cummings was present at an address in the city.
The force adds that the following morning an officer spoke with Mr Cummings’ father at his own request, and he confirmed his son had travelled with his family to the North East and was “self-isolating in part of the property”.
It says the force “deemed that no further action was required. However, the officer did provide advice in relation to security issues”.
In another evening statement, a No 10 spokesperson accuses the Mirror and Guardian of writing "inaccurate" stories about Mr Cummings, including claims that he had returned to Durham after going back to work in Downing Street on April 14.
"We will not waste our time answering a stream of false allegations about Mr Cummings from campaigning newspapers," the spokeperson said.
Asked by a journalist outside his home whether he had returned to Durham in April, Mr Cummings says: "No, I did not."
A host of Tory MPs call for him to resign or for Mr Johnson to sack him.
But the PM, who fronts the daily Downing Street briefing, firmly backs Mr Cummings, saying his aide acted in the best interests of his child, in a way "any parent would frankly understand".
He insists Mr Cummings "acted responsibly, legally and with integrity".
The Prime Minister told a Downing Street press conference: “I have had extensive face-to-face conversations with Dominic Cummings."
He said: "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.
“And I do not mark him down for that.”
Mr Johnson added that “some” of the allegations about Mr Cummings’ behaviour during self-isolation were “palpably false”.
But the PM’s comments fail to quell anger among Tory MPs, opposition parties, scientists and even bishops – one of whom accuses Mr Johnson of treating the public “as mugs”.
Durham councillor Amanda Hopgood says she has written to Durham Constabulary’s Chief Constable Jo Farrell after being made aware of a number of sightings of Mr Cummings in the area in April and May.
Mr Cummings’ parents Morag and Robert defend him in an interview with the New Statesman, with his mother saying the family had been grieving after her brother – Lord Justice Laws – died on April 5 after contracting Covid-19 while ill in hospital, and his father saying he was “disgusted” at the way the press had treated his son during the coverage.
The Rt Revd Nick Baines, Bishop of Leeds, tweeted: “The question now is: do we accept being lied to, patronised and treated by a PM as mugs?”
Education Secretary Gavin Williamson adds his support for Mr Cummings, saying he should not resign “because he has made it clear that he’s broken no rules and he’s broken no laws”.
Gloucestershire’s independent police and crime commissioner Martin Surl says Mr Cummings’ actions make a “mockery” of police enforcement earlier in the lockdown “when the message was very, very clear: stay at home”.
Tory MP David Warburton says his own father died alone as a result of the coronavirus lockdown, and that the Cummings story gives an impression of “double standards”.
Scotland’s First Minister Nicola Sturgeon says she fears Mr Johnson is "putting his political interest ahead of the public interest" and adds that she hopes he will "reflect further" on the matter.
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.
"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."
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