Dominic Cummings has denied he made a second trip to County Durham amid the coronavirus lockdown.
The allegation of a second lockdown breach came after it emerged Mr Cummings travelled from London to Durham in the first week of April with his partner and four-year-old son, after his wife developed coronavirus symptoms and he was self-isolating.
Official guidelines warned against long-distance travel.
The PM pledged his “full support” on Saturday to his under-fire chief adviser.
According to the Sunday Times, the Conservative Party leader told allies he would not throw Mr Cummings “to the dogs” following reports he made the journey to ensure his four-year-old child could be looked after as he and his wife were ill.
However, further allegations surfaced later on Saturday that the former Vote Leave campaign co-ordinator made a second trip to Durham and was seen there on April 19 – five days after being photographed on his return to Westminster.
A second eyewitness told the the Observer and Sunday Mirror they saw the 48-year-old a week earlier in Barnard Castle on Easter Sunday, a popular tourist location 30 miles away from Durham, during the period he was believed to be self-isolating.
Speaking outside his home on Sunday morning, when asked if he had returned to Durham in mid-April, Mr Cummings said: "No, I did not."
The denial comes amid a fierce row over the actions of the Prime Minister’s top aide, with prominent Conservative MPs began calling for the adviser to leave his role.
Downing Street has said it would “not waste time” replying to the fresh allegations from “campaigning newspapers”.
Further pressure was created on Sunday morning when Steve Baker became the first Conservative MP to publicly say Mr Cummings should leave his role.
Mr Baker, a prominent member of the 1922 Committee of backbench Tories, said Mr Cummings “should resign and should resign now".
Speaking to ITV News, Mr Baker said the row surrounding Mr Cummings was diverting attention from "the stuff that really matters to people, like getting out of this lockdown".
He continued: "I was surprised that Dominic broke the rules because it was such an obvious thing he should have avoided.
"I know that I'm watched as a Member of Parliament... and therefore I would have expected somebody in Dominic's position to ensure that he complied with the rules absolutely scrupulously, so I am flabbergasted.
"There's no doubt he does turn out good work - I've often said he is capable of being quite brilliant - and he's really, I think, wrecked his position which is untenable."
ITV News Political Editor Robert Peston said Mr Baker's calls for Mr Cummings' resignation were significant, as he and other members of the pro-Brexit European Research Group played a major role in Mr Johnson becoming prime minister.
Tory MP Sir Roger Gale also described Mr Cummings' position as "untenable".
"I've said very publicly as a father and as a grandfather, I understand any parent's desire to protect their children, but it's not good enough for the prime minister's staff to have one law and everybody else to be asked to obey another," Sir Roger told ITV News.
"I think he's been completely wrong and I think his position is untenable...
"You cannot have one law - you can't say to the public: 'This is what you must do', and then do something wholly different yourself.
"That said, I understand the right of a parent and the desire of a parent to protect a child - I'd want to do the same - but we all have to live by the law and all means Mr Cummings as well as everybody else."
Damian Collins joined the calls for Mr Cummings to go, saying: "The government would be better off without him."
Another Conservative MP, Simon Hoare, called on Mr Cummings to consider his position due to the damage he is doing to the prime minister.
Acting Lib Dem leader Sir Ed Davey said the PM’s judgment would be called into question if he did not give Mr Cummings the chop.
“Surely Boris Johnson must now recognise the actions of his top adviser are an insult to the millions who have made huge personal sacrifices to stop the spread of coronavirus,” said the former energy secretary.
Shadow policing minister Sarah Jones said “people are rightly feeling is it one rule for us and one rule for the people at the top” as Dominic Cummings faced more allegations that he broke lockdown rules.
Asked if he should resign, she told Sky News’s Sophy Ridge On Sunday: “I think what we’ve heard in the last 24 hours suggests some quite serious allegations against the PM’s chief adviser and I think people are feeling rightly angry.
“Millions of people have put their lives on hold, have made huge sacrifices to obey the rules during this period and we’ve seen the heartbreak of people not being able to attend funerals of loved ones, not being able to see their family members as they die.
“And I think people are rightly feeling is it one rule for us and one rule for the people at the top. And I think there are questions to ask, both in terms of what Dominic Cummings did, but also in terms of the response that we saw yesterday, where we seemed to get this rowing back of the rules from minister after minister suggesting that nothing had been done incorrectly.”
The SNP's leader in Westminster, Ian Blackford, wrote on Twitter: “It is clear that Boris Johnson must sack Dominic Cummings.
“When the PM’s top adviser ignores the Government’s instruction to the public not to engage in non-essential travel he has to leave office. Immediately.”
Transport Secretary Grant Shapps has reiterated that Mr Cummings stayed put after travelling to Durham.
He told the Sophy Ridge show: “I don’t have all the times and dates for you but I understand he will have travelled up there towards the end of March and stayed there, remained there for 14 days, didn’t leave the property and isolation, as per the rules and guidance.”
Mr Shapps said he was sure Mr Cummings obeyed social distancing rules.
He said: “You’ll appreciate I wasn’t with them so I can’t tell you exactly what that journey was like, but what I do know is that Dominic Cummings – I saw a clip yesterday of him asking journalists to be spaced two metres apart, so I know he is a stickler for those rules about what to do to make sure you are following the two-metre rule and the like, so I’m sure that they took all the necessary precautions.”
In a statement on Saturday morning, Number 10 said Mr Cummings had travelled to be close to family to seek help looking after his four-year-old child after his wife became ill with coronavirus symptoms – a virus which has seen more than 45,000 people in the UK die after contracting it, according to the latest available data.
According to the papers, 70-year-old retired teacher Robin Lees, of Barnard Castle, said he saw Mr Cummings and his family walking by the River Tees near the town.
He told the Mirror and Observer: “I was a bit gobsmacked to see him, because I know what he looks like.
“It just beggars belief to think you could actually drive when the advice was stay home, save lives. It couldn’t have been clearer.”
The papers also reported that a second unnamed source recalled seeing Mr Cummings in woodland near his family’s Durham property on April 19, recognising him due to him wearing his trademark beanie hat.
Responding to the allegations, a No 10 spokeswoman said: “Yesterday the Mirror and Guardian wrote inaccurate stories about Mr Cummings.
“Today they are writing more inaccurate stories including claims that Mr Cummings returned to Durham after returning to work in Downing Street on 14 April.
“We will not waste our time answering a stream of false allegations about Mr Cummings from campaigning newspapers.”
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