A witness to an alleged lockdown breach by Boris Johnson’s top aide said he has been interviewed by police, as the PM faces pressure from within his own party over Dominic Cummings’ Durham trip.

Robin Lees described having had a “thorough” chat with officers about his claim to have seen Mr Cummings and his family walking by the River Tees on Easter Sunday.

It comes as one minister quit in protest at the senior aide remaining in his Number 10 post, and dozens of Conservative MPs called for Mr Cummings to go despite Cabinet members seeking to protect the controversial adviser.

The Guardian reported that Mr Lees was interviewed on Monday evening, after Mr Cummings gave an extraordinary Downing Street press conference.

The retired teacher, of Barnard Castle, told the newspaper: “They were very thorough, asking every detail of what I saw. They were not in uniform.

“They asked me a lot about my background. They wanted to know exactly what time it was. They wanted to know if it was a clear day and did I know what Cummings looked like.”

In his televised speech on Monday, Mr Cummings said he had driven for half an hour and ended up by a riverbank on the outskirts of Barnard Castle town on Easter Sunday in a trip to test his eyesight before making the long journey back to London the following day.

Returning to the car, an elderly gentleman walking nearby appeared to recognise him, he said, adding that his wife wished the man a Happy Easter “but we had no interaction with him”.

Tuesday saw the first resignation of a minister since Mr Cummings’ trip was revealed in a joint investigation by the Guardian and Mirror late on Friday.

Douglas Ross, the parliamentary under-secretary of state for Scotland, said he could not remain in government after hearing Mr Cummings’ efforts to defend his trip from London to Durham despite the coronavirus lockdown.

Dozens of Conservative MPs have said Mr Cummings should go despite ministers seeking to protect the controversial adviser.

Efforts to defend Mr Cummings’ actions even led Health Secretary Matt Hancock to indicate that fines levied on parents flouting lockdown rules due to childcare concerns could be reviewed.

In his resignation statement Mr Ross, the MP for Moray, said: “I have constituents who didn’t get to say goodbye to loved ones; families who could not mourn together; people who didn’t visit sick relatives because they followed the guidance of the Government.

“I cannot in good faith tell them they were all wrong and one senior adviser to the Government was right.”

Scottish Conservative leader Jackson Carlaw added his voice to dozens of Tory MPs who called for the aide to quit or be sacked by the Prime Minister.

Downing Street insisted the PM had not split the Government by backing Mr Cummings.

But Communities Secretary Robert Jenrick told BBC Radio 2 that his own postbag showed “many people still disagree” with Mr Cummings’ actions.

Critics accused Mr Cummings of undermining the Government’s efforts to curb the spread of coronavirus, which has so far been linked to more than 47,300 deaths in the UK.

Mr Cummings said he had driven to Durham to isolate in a property on his father’s farm because of concerns over who would care for his four-year-old son if both he and his wife were incapacitated by Covid-19.

At the Downing Street press conference, Mr Hancock was asked by a member of the public whether the Government would now review all penalty fines imposed on families travelling for childcare purposes during lockdown.

Mr Hancock said it was a “very good question” and indicated he would consult the Treasury and “look at it”.

Tory MPs confronted with angry correspondence from constituents over the weekend made clear their views on the row, with at least 27 calling for Mr Cummings to go.

Former attorney general Jeremy Wright called for Mr Cummings to quit, saying his actions may technically have been within the rules, but efforts to combat Covid-19 had been due to “people accepting wholeheartedly not just the letter of the restrictions that have been set out, but also their spirit”.

Former chief whip Mark Harper said Mr Cummings “should have offered to resign, and the Prime Minister should have accepted his resignation”.

MPs from the 2019 intake – who owe their positions in part to the campaign Mr Cummings helped run – also called for him to face the consequences of his actions.

Simon Jupp said he felt “anger, disappointment and frustration” and suggested Mr Cummings should consider his position, Craig Whittaker said he should be reprimanded and Elliot Colburn said he should resign for undermining the Government’s messages on controlling the spread of the virus.

Mr Cummings 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.

Cabinet Office Minister Michael Gove said the journey, some 25 miles from where the aide was isolating, was “completely appropriate” because he was “preparing to return to work” by checking he was safe to drive the long trip back to London.

Former Greater Manchester Police chief constable Sir Peter Fahy told BBC Radio 4’s Today programme that Mr Cummings’ trip to Barnard Castle “certainly appears to be against the Highway Code – it’s not the way to test your eyesight, and put potentially other people in danger”.

Experts at the Royal College of Ophthalmologists and Moorfields Eye Hospital said there was little evidence to link Covid-19 to eyesight problems.

Some 71% of Britons believe Mr Cummings broke the lockdown and 59% think he should resign, according to a snap poll of 1,160 adults by YouGov after the aide’s defence, which suggested his statement had turned public opinion further against him.

In other developments:

– Some patients admitted to hospital with Covid-19 are to be given access to the drug remdesivir, which has been shown to shorten recovery time;

– Mr Hancock confirmed that there could be “local lockdowns” in future if the test and trace system identifies coronavirus hotspots;

– Ryanair said it would increase flights to 40% of its normal schedule from July 1;

– John Lewis said it would begin a phased reopening of stores from June 15;

– Health officials sent out a rallying call to Covid-19 survivors to donate their blood plasma as part of a trial to see whether it could be used as a treatment for patients.