Boris Johnson has been hit with a ministerial resignation as Tory anger over the behaviour of the Prime Minister’s senior adviser, Dominic Cummings, boiled over.

Douglas Ross, the parliamentary under-secretary of state for Scotland, said on Tuesday that he was quitting after hearing Mr Cummings’ efforts to defend his trip from London to Durham.

The first resignation over the allegations rocking the Government came as Cabinet Office Minister Michael Gove sought to defend his longstanding ally as having acted in an “entirely reasonable” way, and within the law.

But Mr Ross, the MP for Moray, said that “while the intentions may have been well-meaning”, Mr Cummings’ interpretation of the rules was “not shared by the vast majority of people”.

“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,” the Tory MP wrote.

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

A No 10 spokesman said Mr Johnson “regrets” Mr Ross’s decision to quit.

Downing Street insisted the PM had not split the Government by backing Mr Cummings amid concerns over how the decision will affect the public, police and health workers during the pandemic.

In an extraordinary press conference for an adviser, Mr Cummings 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.

Downing Street declined to answer numerous unresolved queries, but Mr 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.

“It’d have been entirely within his right to return to work that day on the basis of the advice he had been given, that’s my understanding, so that drive was completely appropriate,” Mr Gove told BBC Radio 4’s Today.

Former Greater Manchester Police chief constable Sir Peter Fahy said officers had become “frustrated” by the fiasco, which may hinder policing with the rules “now very confused”.

And he suggested 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”.

He also said “it may well be that absolutely he’d have been turned back” by officers if they stopped him during the drive north from London in March.

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.

Housing Secretary Robert Jenrick told BBC Radio 2 that his own postbag showed “many people still disagree” with Mr Cummings’ actions and could not rule out further resignations.

NHS Confederation chief executive Niall Dickson said: “Because of the way this story has unfolded, there is certainly concern among our members, health leaders, that it could damage staff and public confidence in official guidance.”

In other developments:

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

– 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;

– Leaders of opposition parties – although not Labour’s Sir Keir Starmer – were discussing how to respond to the Cummings row.

The political storm surrounding Mr Cummings’ press conference in Downing Street’s garden overshadowed Mr Johnson’s announcement that all shops in England will be able to open next month if they can protect shoppers and workers.

Mr Cummings said he and his wife, journalist Mary Wakefield, drove to Durham to stay in a cottage on his father’s farm because of concerns about childcare for their four-year-old son if they were incapacitated by coronavirus and also over fears about the safety of their London home.

He declined to apologise and said he did not regret his actions when he outlined how he drove from his home in London to County Durham during the lockdown.

Despite efforts by ministers to draw a line under the row, anger on the Tory back benches persisted.

Tory MP Simon Jupp suggested that Mr Cummings should consider his position, saying he has felt “anger, disappointment and frustration” during the “deeply unhelpful distraction”.

William Wragg, Tory chairman of the Commons Public Administration and Constitutional Affairs Committee, said it was “humiliating” to see ministers defending Mr Cummings.

“We cannot throw away valuable public and political goodwill any longer,” he said.

Veteran Conservative MP Sir Roger Gale said the backbench 1922 Committee should tell Mr Johnson that Mr Cummings has to go.