In a move that threatens to engulf Mr Johnson’s premiership, the Met announced on Tuesday the Prime Minister was fined for attending his own birthday bash in the Cabinet Room of No 10 on June 19 2020. It makes him the first sitting prime minister to have been found to have broken the law.
The chancellor was also fined on a day that saw all the main opposition parties demand his immediate resignation.
Making a statement in Chequers, Mr Johnson insisted he did not think this actions would break the laws, despite making them himself.
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He said: “I’ve paid the fine and I once again offer a full apology and in the spirit of openness and humility I want to be completely clear about what happened on that date.
“My day began shortly after 7am and I chaired eight meetings in Number 10, including the Cabinet committee deciding Covid strategy.
“I visited a school in Hemel Hempstead, which took me out of Downing Street for over four hours and amongst all these engagements on a day that happened to be my birthday, there was a brief gathering in the Cabinet Room shortly after 2pm lasting for less than 10 minutes, during which people I work with kindly passed on their good wishes.
“And I have to say in all frankness at that time it did not occur to me that this might have been a breach of the rules.
“Of course I take full responsibility for everything, but don’t forget the Downing Street is about, you know, 15,000 square feet. It’s got a lot of officials working in it – hundreds and hundreds of officials – I couldn’t be everywhere at once.
The Prime Minister said he “spoke in completely good faith” when he repeatedly said all guidelines were followed in Downing Street as it did not occur to him that he was in breach of the rules.
Mr Johnson explained: “When I said that I spoke in completely good faith because as I’ve said to you just now I… at the time that I was standing up for nine minutes in the Cabinet Room where I work every day, it didn’t occur to me that, as I say, that I was in breach of the rules.
“I now humbly accept that I was.
“But I think the best thing I can do now is, having settled the fine, is focus on the job in hand. That’s what I’m going to do.”
The fines raise difficult questions for the pair, with both men having previously denied breaking the rules.
Mr Sunak told MPs on December 7 last year he “did not attend any parties”, while the Prime Minister insisted all “rules were followed”.
By convention, a minister found to have misled parliament is expected to resign or face being sacked.
The Labour leader Keir Starmer has now called for both men to resign, saying “Britain deserves better, they have to go.”
He said: “The British public made the most unimaginable, heart-wrenching sacrifices, and many were overcome by guilt. Guilt at not seeing elderly relatives, not going to funerals or weddings, or even seeing the birth of their own children.
“But the guilty men are the Prime Minister and the Chancellor. They’ve dishonoured all of that sacrifice, they’ve dishonoured their office.
“This is the first time in the history of our country that a Prime Minister has been found to be in breach of the law, and then he lied repeatedly to the public about it.”
The announcement sparked a wave of condemnation from across the opposition parties, and comes after at least 30 more fines relating to breaches of Covid-19 regulations in Downing Street and Whitehall were announced.
This takes the total number of fines issued to more than 50, including for the Prime Minister’s wife, Carrie Johnson.
Mrs Johnson’s spokesperson said: “In the interests of transparency, Mrs Johnson can confirm she has been notified that she will receive a fixed-penalty notice.
“She has not yet received any further details about the nature of the FPN.”
A poll of 2,464 adults by YouGov has revealed 57 per cent of responders think Mr Johnson should resign, while 30 per cent say he should stay.
The same proportion said Rishi Sunak should also resign as Chancellor for being at the same event.
And 75 per cent of responders said they thought Mr Johnson knowingly lied to Parliament about whether he broke lockdown rules, with just 11 per cent saying he did not.
The SNP's Westminster leader Ian Blackford MP demanded both men resign, and claimed it cannot be “one rule for the Tories, and another for the rest of us”.
He said: "The Prime Minister repeatedly misled Parliament, lied to the public and at times even simply laughed it off - taking the public for fools.
"In reality, Johnson and Sunak have overseen one of the biggest lockdown breaches that has led to the Metropolitan Police issuing a staggering number of fines for rule-breaking.
"The Prime Minister and Chancellor's positions are untenable. They have insulted the millions of people who faithfully followed the rules that they set – even when that meant the heartbreak of not being present for the births, marriages or the deaths of loved ones.
"If Johnson and Sunak have a shred of dignity they will finally do the right thing and resign now. If they refuse, then it is on Tory MPs to step up and act to remove them from office to restore public trust."
The First Minister added: “Boris Johnson must resign. He broke the law and repeatedly lied to parliament about it.
"The basic values of integrity and decency - essential to the proper working of any parliamentary democracy - demand that he go.
“And he should take his out of touch chancellor with him.”
Liberal Democrat leader Sir Ed Davey joined the chorus demanding the Mr Johnson go.
He tweeted: “This is a government in crisis neglecting a country in crisis.
“Parliament must be recalled for a vote of No Confidence in the Prime Minister.”
Covid-19 Bereaved Families for Justice have said there is “simply no way” the Prime Minister and Chancellor can continue in post , claiming the pair “broke the law” and “took us all for mugs”.
Spokesman Lobby Akinnola, said: “There is simply no way either the Prime Minister or Chancellor can continue.
“Their dishonesty has caused untold hurt to the bereaved.
“Not only that, but they have lost all credibility with the wider public, which could cost lives if new variants mean restrictions are needed in the future.
“If they had any decency they would be gone by tonight.”
Tory MPs were less quick to demand Mr Johnson go, instead pointing to the Ukraine crisis as a reason to keep him.
Scottish Tory leader Douglas Ross has said it “wouldn’t be right” to remove the Prime Minister during the Russian invasion of Ukraine.
Mr Ross, previously one of the leading voices calling for the Prime Minister’s resignation over the partygate saga, said: “The public are rightly furious at what happened in Downing Street during the pandemic. I understand why they are angry and share their fury.
“The behaviour was unacceptable. The Prime Minister now needs to respond to these fines being issued.
“However, as I’ve made very clear, in the middle of war in Europe, when Vladimir Putin is committing war crimes and the UK is Ukraine’s biggest ally, as President (Volodymyr) Zelensky said at the weekend, it wouldn’t be right to remove the Prime Minister at this time.
“It would destabilise the UK Government when we need to be united in the face of Russian aggression and the murdering of innocent Ukrainians.”
Alister Jack, Secretary of State for Scotland, said: “The Prime Minister has, rightly, apologised and accepted responsibility for actions which he knows have angered a great many people.
"However, he remains the right person to lead this country at such a crucial time and we need to get behind him so that he can focus on dealing with the appalling situation in Ukraine and on delivering for everyone in this country.”