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Dominic Cummings is under increasing pressure from Tory MPs to resign after offering “no credible justification” for his decision to travel around the country during the coronavirus lockdown.

Boris Johnson’s senior aide has argued that his journey to Durham in March was justified as he sought to protect his family’s health.

But many questions remained unanswered, including over his subsequent drive to Barnard Castle which he said was to test his eyesight after it was affected by Covid-19.

On Tuesday morning Douglas Ross resigned as a junior minister in the Scotland Office saying he could not ‘in good faith’ defend Cummings’ actions.

Mark Harper, a former Tory chief whip, said today Cummings “should have offered to resign, and the prime minister should have accepted his resignation”.

Johnson has defended the actions of his longtime adviser and has so far ignored demands he be fired.

A YouGov poll published today showed 59% of people believed Cummings should quit.

Mark Pawsey, the Tory MP for Rugby & Bulkington, said Cummings had “acted against the spirit of the lockdown” and the prime minister “should now ask for his resignation”.

Simon Jupp, the MP for East Devon, this morning said he would have considered resigning had he taken the same actions as Cummings.

“We are all making significant sacrifices and coping with situations we couldn’t imagine just a few months ago,” he said.

“Many of us, including myself, have lost people in our lives and haven’t been able to see family and friends. It’s been incredibly tough for everyone.”

William Wragg, the MP for Hazel Grove, said it had been “humiliating” and “degrading” to see ministers defend Cummings.

“This is a time of national emergency and our focus must be unrelenting. We owe it to the nation,” he tweeted.

Michael Gove, an ally of Cummings, said Cummings’ journey to Barnard Castle, some 25 miles from where the aide was isolating, was “completely appropriate”.

The Cabinet Office minister told Sky News this morning it was justified because he was “preparing to return to work” by checking he was safe to drive the long trip back to London.

But Harper said: “As for Mr Cummings’ trip to Barnard Castle on April 12, there is no credible justification for this.

“An apology should have been made and a level of regret expressed. I was disappointed that Mr Cummings did neither.”

He added that he would “expect an adviser who had damaged the credibility of the government’s central message so badly and had become the story to consider their position”.

Harper, who last year ran for the Tory leadership against Johnson, concluded: “Difficult times are ahead, which will require the government to be able to deliver clear and credible public health messages. In the interests of us all, I hope this will still be possible.”

Alec Shelbrooke, the MP for Elmet and Rothwell, said Cummings’ position was now “untenable”.

In an email to a constituent seen by HuffPost UK, Shelbrooke said the Barnard Castle trip was not “justifiable.

“Many of my constituents, myself included, have made difficult decisions over the last few months,” he said.

“I understand these are incredibly difficult times. I believe the decision Mr Cummings made to travel to Barnard Castle was wrong and outside of the regulations. For that reason, I think his position is untenable.”

Tory donor Alexander Temerko defended Cummings. “In any society, there are people who are ready to turn any good thought, idea, regulation or even faith, into a state of religious fanaticism,” he told HuffPost UK.

“Dominic Cummings is not a fanatic; he is a reasonable person, and he did what any man should have done. Nothing outright illegal – and rather honest and courageous.

“Of course, I understand the anger levelled at him, by those who made self-isolation and isolation of loved ones a form of asceticism. In truth, I think it is absolutely wrong that 95% of the capable working population is forced to stay at home indefinitely.

“Today, it is no longer self-isolation, it is self-destruction – of the national economy and our mental health.”