Boris Johnson has become the first sitting prime minister to break the law after he was fined by police over the Partygate scandal.
The Tory leader offered a "full apology" for breaching his own Covid laws but refused to resign even though millions of Brits had stuck to the rules.
His wife Carrie and Chancellor Rishi Sunak were also issued fixed penalty notices for attending the PM's lockdown-busting birthday party on 19 June, 2020.
The Met Police shattered the fragile peace the PM had struck with his own backbenchers when it announced 30 more fines had been issued yesterday, taking the total to 50.
Scotland Yard confirmed its investigation into at least a dozen social gatherings at the heart of Government, first revealed by the Daily Mirror, was continuing.
Tonight jittery Tory MPs were holding their own counsel over the PM's future as it became clear he could yet face further fines, potentially dragging the controversy on for weeks.
He is alleged to have attended at least six of the social gatherings the Met is investigating, including a party in his own flat.
Mr Johnson, speaking at the end of a dramatic day from his grace and favour residence Chequers, admitted that he had paid the police fine.
"It did not occur to me that this might have been a breach of the rules. But of course the police have found otherwise and I fully respect the outcome of their investigation," he said.
"I understand the anger that many will feel that I myself fell short when it came to observing the very rules which the Government I led had introduced to protect the public.
"I accept in all sincerity, that people had the right to expect better".
Mr Johnson is also accused of misleading Parliament - usually a resigning matter - four months ago when he vowed solemnly: "All guidance was followed completely in No10."
But he denied lying to the House of Commons, claiming: "I spoke in completely good faith".
The under-fire PM faced calls to stand down from bereaved families who lost loved ones during the pandemic and the leaders of all the main opposition parties.
Keir Starmer, a former director of public prosecutions, said the police's decision marked the "first time in the history of our country that a PM has been found to be in breach of the law".
The Labour leader accused Mr Johnson of "repeatedly" lying about what happened behind the famous black door of No 10.
He also argued that the Tory leader and Mr Sunak had "dishonoured" the sacrifices made by millions of Brits who did follow the rules during the pandemic.
"The British public made the most unimaginable, heart-wrenching sacrifices, and many were overcome by guilt," he said.
"But the guilty men are the Prime Minister and the Chancellor. Britain deserves better, they have to go."
A snap YouGov poll found that 57% of people think he should quit, while 30% want him to stay. A staggering 75% say he knowingly lied about breaching the law.
The Liberal Democrats called for the PM to recall Parliament so that he can face questions - and a confidence vote - from MPs over the scandal.
Plans to end Easter recess early because of potential chemical weapons use in Ukraine were reportedly junked so Mr Johnson could avoid awkward questions about Partygate.
No10 confirmed the £100 fine, which is halved to £50 if paid within 14 days, related to the PM's surprise 56th birthday party on June 19, 2020, when indoor gatherings were banned.
The event, said to have been organised by wife Carrie, was held in the Cabinet Room in No10 where staff ate picnic food and sang 'Happy Birthday'.
PM ally Conor Burns notoriously tried to defend the PM over the bash by claiming he had been "ambushed by cake" - even though officials denied one had been present.
But the Mirror was told that police had a photograph of Mr Johnson holding a can of an Estrella beer at the bash, taken by the PM's taxpayer-funded photographer.
Mrs Johnson's spokeswoman claimed she had only briefly attended the event briefly with her newborn baby Wilf in her arms.
The PM said he now "humbly" accepted he had broken the rules - but that he now wanted to move on.
"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."
Grieving Covid families said Mr Johnson and his Chancellor should stand down as they had caused "untold hurt" and lost credibility with the wider public.
Lobby Akinnola, spokesperson for Covid-19 Bereaved Families for Justice, said: "After everything that’s happened it’s still unbelievably painful to know that the Prime Minister was partying and breaking his own lockdown rules - while we were unable to be at our loved ones' sides in their dying moments, or in miserable funerals with only a handful of people - because we were following the rules.
"The fact that the Prime Minister and his Chancellor then lied about it, and would have continued to do so if the police hadn’t intervened, is truly shameless. They broke the law. But even worse, they took us all for mugs.
"There is simply no way either the PM or Chancellor can continue."
Just a handful of the PM's most loyal allies gave him their public backing, with many other Tory MPs waiting to see how the coming weeks play out before deciding whether to oust him.
One loyalist, Michael Fabricant, sparked fury by claiming that hard-working teachers and NHS staff had been drinking on the job during the pandemic.
He said: "I don't think at any time he thought he was breaking the law... he thought just like many teachers and nurses who after a very long shift would go back to the staff room and have a quiet drink".
But although the PM's handling of the Ukraine crisis has bought him some time with his own party, many Tories warned he could not be the leader to take them into the next election.
Long-time Tory critic Sir Roger Gale claimed it was not the right time to unseat the PM - but added: "I don't think the PM will lead us into the next election..."
Another senior MP added: "It’s clearly game over - it’s just a question of when, not if."
Tory rebels need to gather 54 letters of no confidence in the PM to trigger a leadership contest - though he is likely to be bolstered by the lack of an obvious successor.
Mr Johnson faces a bumpy few weeks ahead with the prospect of more fines and the May local elections giving voters a chance to have their views known.
Many of his allies had believed he had he was likely to avoid getting fined - so the Met's decision came as a shock to No 10.
