Police allegedly approached Mr Johnson’s chief aide to explain government advice against non-essential travel after he was spotted some 375km north of the capital.
No 10 confirmed that Mr Cummings was self isolating after developing coronavirus symptoms at the end of March – shortly after he was seen running out of Downing Street following the news that Mr Johnson and Matt Hancock had been struck down by the virus.
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Downing Street was repeatedly asked by journalists about Mr Cummings’s whereabouts during this period but the prime minister’s official spokesman would only say that he was “in contact with No 10”.
A joint report by the Daily Mirror and The Guardian has now claimed that Mr Cummings had travelled to his parents’ home in County Durham – despite government instructions to stay at home.
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Mr Cummings was reportedly seen at the property with his young son on 5 April, the same day the prime minister was admitted to hospital with coronavirus.
The sighting occurred on the same day that Scotland’s chief medical officer Catherine Calderwood stepped down after breaching lockdown rules to visit her second home.
Professor Neil Ferguson, one of the government’s scientific advisers, also had to resign from his post after flouting guidance to visit his married lover.
Sir Ed Davey, the acting Liberal Democrat leader, said: “If Dominic Cummings has broken the lockdown guidelines he will have to resign. It’s as simple as that.”
SNP Westminster leader Ian Blackford said his position was “untenable”, adding: “Boris Johnson must now show Domninic Cummings the door – otherwise people will conclude the Tories were so keen to leave lockdown because they weren’t following the rules in the first place.”
A Labour spokesperson said there must be a “very swift explanation” for his actions.
“The government’s guidance was very clear: stay at home and no non-essential travel,” a spokesperson said.
“The British people do not expect there to be one rule for them and another rule for Dominic Cummings. No 10 needs to provide a very swift explanation for his actions.”
Downing Street did not respond to repeated requests for comment, but a friends of Mr Cummings said: “He isn’t remotely bothered by this story, it’s more fake news from the Guardian.
“There is zero chance of him resigning.”
Lockdown rules were unveiled by the prime minister on 23 March, which said: “You should not be visiting family members who do not live in your home.
“The only exception is if they need help, such as having shopping or medication dropped off.”
On 27 March, Mr Johnson was revealed to be suffering with coronavirus. Mr Cummings was seen dashing out of No 10 shortly after the announcement.
His wife Mary Wakefield recently wrote in The Spectator that he began to feel unwell around 24 hours after he rushed home to be with her.
But a member of the public reportedly spotted Mr Cummings in Durham on 31 March and complained to the police.
A spokesperson for Durham Constabulary said: “On Tuesday, 31 March, our officers were made aware of reports that an individual had travelled from London to Durham and was present at an address in the city.
“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 guidelines around self-isolation and reiterated the appropriate advice around essential travel.”
A neighbour then reportedly spotted Mr Cummings at his parents’ house on 5 April after hearing Abba blasting from the garden.
The neighbour, who has not been named, said: “I got the shock of my life, as I looked over to the gates and saw him.
“There was a child, presumably his little boy, running around in front. I recognised Dominic Cummings, he’s a very distinctive figure.”
They added: “I was really annoyed. I thought it’s okay for you to drive all the way up to Durham and escape from London.
“I sympathise with him wanting to do that but other people are not allowed to do that. It’s one rule for Dominic Cummings and one rule for the rest of us.”
Mr Cummings was next seen in Downing Street on 14 April, where he was photographed entering the building clutching a packed lunch.