For the past week, the British Prime Minister has been buffeted by multiple claims that social events were held inside 10 Downing Street in the runup to Christmas 2020, when the country was under coronavirus regulations that outlawed such activities.

The controversy deepened markedly on Tuesday with the emergence of a video broadcast by CNN affiliate ITV News from the time that appears to show Number 10 officials joking about the parties during a rehearsal for televised press briefings.

Downing Street's position, before and since the video was leaked, is this: "There was no Christmas party and coronavirus rules had been followed at all times." Johnson has announced an internal inquiry and on Wednesday, one of the officials in the video -- the prime minister's former spokeswoman Allegra Stratton, resigned as a government adviser.

After speaking to multiple officials, CNN has established that social gatherings were indeed held on the two days in question -- November 27 and December 18 -- and has confirmed a Daily Mirror report that Johnson himself gave an impromptu speech at the first one. Sources also confirmed reports that secret Santa gifts were exchanged on December 18.

The accusations are damaging for the government as it seeks to enforce tougher coronavirus restrictions. On Wednesday, Johnson announced England would be tightening restrictions -- called "Plan B" -- due to rising Omicron cases: From Friday, face coverings will again become mandatory in most public venues, including cinemas. And from next week people will be advised to work from home and present a "Covid Pass" proving vaccination or recent recovery to enter nightclubs and other venues with large crowds.

"It's becoming increasingly clear Omicron is growing faster than the previous Delta variant," Johnson said, adding that the more than 500 cases identified in the UK are "almost certainly an underestimate."

Against this backdrop of rising cases, the charge of government hypocrisy is potent -- last year it forced the resignation of a key aide to Johnson, and the health secretary in June this year, in separate scandals that were also characterized by claims of double standards.

Allegra Stratton, Boris Johnson's former press secretary, resigned later on Wednesday.

The video itself is the most damaging element of the story. It shows Stratton rehearsing for a televised news conference on Tuesday December 22, being asked by colleagues whether a party took place "on Friday night" -- which would have been December 18, the date of the second alleged party. Stratton says, "I went home." She then laughs and smiles. "Hold on. Hold on. Um. Er. Ah."

She also said in the clip, shown by ITV, "This fictional party was a business meeting and it was not socially distanced."

Until now, the government had maintained its line that no party had taken place. But on Wednesday, referring to the video, Johnson apologized in parliament on Wednesday "unreservedly for the offense that it has caused." He added: "I have been repeatedly assured that there was no party and no rules were broken." He announced that his Cabinet Secretary will be investigating the events of December 18 -- the date of the second alleged party and not the event at which CNN has been told he spoke -- and would report back.

Johnson said that if rules were broken "there will be disciplinary action for all those involved." He added that he was "furious" when he saw the video.

Stratton, who was serving as the Prime Minister's COP26 spokeswoman, said her comments had become a "distraction" in the fight against coronavirus, and offered her resignation on Wednesday afternoon. "I understand the anger and frustration that people feel," she said in a statement on camera.

Allegra Stratton in the leaked video, published by ITV News.

"To all who lost loved ones, endured intolerable loneliness and struggled with your businesses -- I am sorry and this afternoon I offered my resignation to the Prime Minister."

Johnson paid tribute to Stratton at a press conference later Wednesday. The prime minister told reporters that while he can "totally understand how infuriating" the leaked video was, Stratton had been a "fine colleague" who "achieved a great deal in her time in government" particularly as COP26 spokesperson.

Johnson also maintained that to the best of his knowledge, no Covid-19 restrictions had been broken. When grilled by reporters on whether people would heed the latest restrictions after members of his team laughed and joked about the coronavirus, Johnson replied: "The British public can see the vital importance of the medical information that we're giving."

Conservative Party chairman Oliver Dowden said last week, "My focus has not been on having conversations about what may or may not have happened over a year ago in Downing Street. I have, however, been assured by what the Prime Minister has said, which is that the rules at all times were obeyed in Downing Street."

CNN approached multiple other government officials for comment but had no reply.

The video, which could be dismissed as a jovial knockabout between advisers never intended to be broadcast, undermines Downing Street's key claim that no rules were broken. In addition to the alleged party on December 18, there was another earlier gathering for departing senior adviser Cleo Watson, which took place on November 27. CNN has attempted to contact Watson.

Both gatherings took place while London was under strict lockdown measures and social gatherings were banned. The day before the December 18 gathering, the government even sent a tweet warning "you must not have a work Christmas lunch or party, where that is a primarily social activity."

Millions of Britons who'd hoped to enjoy some semblance of normality after a year of missing family gatherings were unable to celebrate Christmas together in 2020, precisely because of the government-imposed limits.

A family gather around the television in Liverpool, northwest England to watch Boris Johnson give a televised message to the nation on January 4, 2021.

More than 145,000 people have died from Covid-19 in the UK, and the latest revelations about the video sparked outrage from relatives of those who died in the pandemic.

"There are simply no words to describe how upsetting and shameful it is to then hear Boris Johnson's team laughing about breaking the rules they had made, whilst others followed them and could only say goodbye to their loved ones through a screen," the Bereaved Families group said in a statement. "It's the behaviour of people who think they're above us."

For obvious reasons, this is a deeply damaging moment for a PM who has been accused many times of acting as though there is one rule for himself and his inner circle and the citizens he governs.

In order to understand what happened on the days of the 2020 parties, CNN talked with multiple current and former government officials who say that staff gathered in Downing Street, had drinks and behaved in a way that was at the time unimaginable to the rest of the country. What is less clear is how those gatherings came to pass.

Boris Johnson receives his first dose of a AstraZeneca/Oxford Covid-19 vaccine at St. Thomas' Hospital in London on March 19, 2021.

Based on the conversations CNN has had with multiple current and former government officials, several events were held in Downing Street at the various stages of lockdowns and Covid restrictions. At this time, many officials were still working at the Prime Minister's official residence as "key workers" -- a designation used by the UK government for categories of employees that were exempt from working-from-home requirements.

One senior Number 10 official, who is not a Johnson ally and was present at the gatherings in question explained, "Obviously this looks bad, but from our perspective, we were working 18-hour days. It was entirely normal that we would have drinks on our desks." They claim that these were not organized parties as such, but semi-spontaneous gatherings at the end of stressful workdays between people who, because of their jobs, had to mix anyway.

According to three current and former officials, who were present, Prime Minister Johnson gave a short impromptu speech at Watson's leaving party on November 27, 2020. Of the PM's speech, they say that Johnson was on the way to his flat inside Downing Street, saw that staff had gathered and delivered a few words before going upstairs. Downing Street has denied any rules were broken.

From the numerous conversations CNN has had with people who were present at these events or have a detailed understanding of what went on, it remains very murky as to how this happened.

Current and former officials, appreciating how damaging the revelations of gatherings are, say it was standard practice that staff exchanged secret Santa gifts at Christmas. "To us, Downing Street was an island where we had to work and lockdown wasn't happening in the same way it was for the rest of the country," one current No. 10 official says.

In a single quote, this lays bare the problem Johnson now faces. It is an admission that the rules simply didn't apply to a select group of people who worked directly for the PM, who himself was hospitalized for a week and spent time in intensive care with the virus.

Boris Johnson with his former chief adviser Dominic Cummings, who become an arch Johnson critic since leaving the government.

None of which means this is good for Johnson. The best-case-scenario for the government is that the public accepts staff in No. 10 were working in a small, crowded house at a time a virus was rampant because they had no alternative and, after a hard day's work, they had a drink before returning to duty (on both days, operational communications were sent to journalists beyond the hours that the parties are alleged to have concluded.)

However, the video of Stratton joking about the party has turned even Johnson's allies against him.

A former official and ally of Johnson, who was not present at the events in question said that they had carried on working through a time of crisis while many others fled London to work from home. The official said the people seen in the video would have to be sacked.

Another Johnson supporter, who remains close with the PM, said "it's hard to see how someone doesn't walk for it."

Johnson faces some existential questions about his and his government's competence and how they created an almighty trust issue at the heart of government. And, alarmingly, someone who was close enough to be in a room with Johnson's most trusted inner circle, who has some of the highest security clearance possible, leaked a video damaging to a sitting Prime Minister.

Conservative lawmakers have long privately complained that Johnson has surrounded himself with advisers who were inexperienced in working in such high office.

Johnson came into office promising to deliver on the "people's priorities" and actively courted the votes of those exhausted by politics as usual.

A less charitable explanation is that he believes it is acceptable for his staff to ignore the laws that his own government legislates.

It comes at a time when public trust in his government has dropped because of numerous sleaze scandals. It was only weeks ago that Johnson tried to save a member of his own party who had been involved in a lobbying scandal. There is an ongoing investigation into how the PM paid for a refurbishment of his Downing Street flat, which he has insisted he paid for.

The Prime Minister who won a majority in 2019 as the revolutionary candidate willing to upend the cosy Westminster establishment currently looks like a man who is neck-deep in the worst excesses of political sleaze.

And if history tells us anything, individual scandals don't sink a leader; but a tidal wave of sleaze can drown them. Johnson might have passed the point at which it's possible to keep his head above water.