Speaking at the Scottish Parliament, she said: “People across the country are aghast at the revelations about Downing Street’s conduct.
“It appears not just one isolated breach but serial breaches of guidance that people were following through the painful sacrifices right throughout this pandemic and a Prime Minister who is apparently not being truthful about his knowledge of these matters.
“I don’t think it will surprise anybody to hear my view that the office of Prime Minister would be greatly enhanced by Boris Johnson’s departure from it but more importantly I think at this moment in time the interests of the United Kingdom would be enhanced by that as well.”
A growing number of Tories have lashed out at the Prime Minister amid the fresh allegations he attended a rule-breaking drinks party in Downing Street during the first national lockdown.
The news has drawn fury from MPs and other prominent Conservative figures, while former No 10 chief of staff Lord Barwell ridiculed the PM on social media – suggesting it would not take an investigation for Johnson to figure out if he had been part of the gathering.
Police are in contact with the Cabinet Office over the claims, which came to a head on Monday night.
It has been alleged that a senior aide to Mr Johnson organised a “bring your own booze” event in the garden behind No 10 on May 20, 2020.
Martin Reynolds, the Prime Minister’s principal private secretary, sent an email to more than 100 Downing Street employees inviting them to the evening gathering, ITV reported.
Mr Reynolds said they should “make the most of the lovely weather”, despite England being under tough coronavirus restrictions banning groups from meeting socially outdoors when the message was sent.
The allegations are being examined by senior official Sue Gray as part of her investigation into claims of lockdown-busting parties in Whitehall and Downing Street.
Robbie Moore, Conservative MP for Keighley, said he was incensed by the leaked email.
He said: “The email from Martin Reynolds infuriates me. As I have previously made clear, I am deeply angered by the thought of any illegal gatherings, parties or social events which took place at Downing Street whilst the rest of the country was in lockdown and following the rules.
“The whole country has sacrificed so much during the pandemic and many are quite rightly furious.
“Investigations into these events at Downing Street during lockdown are ongoing, but answers are needed quickly.
“If any event took place that was in breach of the rules, the strongest disciplinary action must be taken, and I will continue to follow any revelations on this closely.”
And Philip Davies, Tory MP for Shipley,said that the allegations were “impossible to justify and defend”.
“It is clearly unacceptable for anyone in Government to be flouting the laws that they had imposed on everyone else,” he said.
“For some of the people who imposed those damaging laws to have been ignoring them will be a bitter pill to swallow for those who faithfully followed them even at huge personal cost, and is impossible to justify and defend.”
Elsewhere, Ruth Davidson, the former leader of the Scottish Conservatives, said “people are (rightly) furious”.
Responding to health minister Ed Argar’s comments on the matter on Tuesday morning, in which he said he understood people were angry but pointed to an ongoing investigation by Ms Gray, she tweeted: “This line won’t survive 48 hrs.”
“Nobody needs an official to tell them if they were at a boozy shindig in their own garden,” she said.
“People are (rightly) furious. They sacrificed so much – visiting sick or grieving relatives, funerals. What tf (sic) were any of these people thinking?”
Lord Barwell, who was chief of staff to Theresa May, also said on Twitter: “Let me put this politely: it is not ‘entirely clear’ why the Prime Minister needs to wait for Sue Gray’s report to find out if he went to a party in his own garden.”