Boris Johnson has "categorically" denied he was warned in advance a drinks event in the Downing Street garden risked breaking Covid lockdown rules.
The claim, by the prime minister's former senior adviser Dominic Cummings, relates to one of a number of alleged parties involving people linked to the government. An investigation by senior civil servant Sue Gray is under way.
What was the drinks party attended by the PM?
Mr Johnson has apologised to MPs for attending a drinks party in the Downing Street garden, during the first lockdown, in the spring of 2020.
But he had not been notified in advance of the party, he said, and had "believed implicitly" it had been a work event.
He had spent 25 minutes thanking staff, before returning to his office, the prime minister said.
Details of the 20 May event - to which up to 100 people had been invited to "bring your own booze" - were revealed by ITV News.
About 30 people are understood to have attended.
What were the Covid rules on 20 May 2020?
Throughout the pandemic, there has been a mixture of legal restrictions and guidance about how people should behave.
Legal restrictions are underpinned by sanctions such as fines, or prosecution.
Guidance is strong advice set out by the government.
But unless it is backed up in law, there are no fines or prosecution for breaking guidelines.
Legal restrictions at the time said that people could not leave their homes - or be outside the place they live - without a reasonable excuse, which included work (where you couldn't work from home), exercise and getting things like food and medicine.
For people who broke these rules, the police in England could fine them £100 for the first offence which could then double for each further offence up to a maximum of £3,200.
It would be difficult to see how the Downing Street event would have been in line with these rules, argues barrister Adam Wagner.
"If you were doing something which wasn't necessary for work then you weren't outside of your house [with] a reasonable excuse and you were potentially committing a criminal offence."
However, Mr Wagner added that as the prime minister and his wife live in Downing Street they would not have technically left their home to attend the party.
Mr Wagner is involved in a case to bring a judicial review against the Met Police for not investigating the alleged parties.
The law also banned gatherings in a public place of more than two people, unless they were all members of the same household or the gathering was "essential for work purposes". However, lawyers have noted that Downing Street is not a public place.
As well as legal restrictions, guidance not enforced by the risk of the prosecution was also in place.
On the day of the party the government's own Twitter account summarised the guidance by saying that gatherings must be limited to two people outside.
But as with the law, the detailed guidance always allowed for people to work where that had to involve gathering with other people.
Mr Johnson told MPs that as a "work event", the gathering was technically within the guidance.
But by 20 May 2020 there was detailed guidance on what to do in offices and other similar settings. When it came to workplace gatherings, it said: "Workers should try to minimise all meetings and other gatherings in the workplace."
The "working safely during coronavirus" guidelines also said only "absolutely necessary participants should attend meetings and should maintain 2m separation throughout". Generally, workers were told to "reduce the number of people you spend time with in a work setting".
There is nothing in the guidelines that would suggest that drinking, socialising or other types of work event along these lines would have been allowed.
What about the rules around other alleged parties?
A number of other gatherings are alleged to have taken place in Downing Street during 2020.
Two leaving parties took place on 16 April and went on until the early hours. Legal restrictions at the time banned gatherings indoors between different households (unless in a support bubble). There was an exemption for "work purposes" but this did not mention socialising at work. Boris Johnson did not attend and was at his official country residence, Chequers, at the time
One was alleged to have taken place on 27 November - a leaving party for Cleo Watson, a former aide to Dominic Cummings.
At this point a national lockdown was in force and indoor gatherings with other households were not allowed (unless for work).
This lockdown ended on 2 December and England returned to the tier system of restrictions where there was a specific prohibition on organising an indoor gathering of more than 30 people.
When a Downing Street Christmas quiz took place on 15 December, London was under tier 2 restrictions. These rules banned two or more people from different households from meeting indoors, unless "reasonably necessary" for work purposes.
The same rule would have applied when leaving drinks were held at the Cabinet Office for Kate Josephs, the outgoing head of the Covid taskforce, on 17 December. It would also have applied to a party - which was joked about in a leaked Downing Street press conference video - alleged to have taken place on 18 December. At this time, London had been moved to tougher tier 3 restrictions.
Furthermore, the government's guidance for the Christmas period specifically said: "Although there are exemptions for work purposes, you must not have a work Christmas lunch or party, where that is a primarily social activity and is not otherwise permitted by the rules in your tier."
This same line was also tweeted out by the official gov.uk account on 17 December 2020 in response to a question from a member of the public about whether Christmas parties were allowed in the workplace.
How has the PM responded to the allegations?
Allegations of Downing Street parties first surfaced in the Daily Mirror, when it reported in early December that a party had been held in Number 10 during the 2020 Christmas period.
In response, Mr Johnson told MPs that that "all guidance was followed completely in No 10".
However, after the Downing Street press conference video was leaked - which appeared to confirm a Christmas party took place - Mr Johnson said he understood the public anger.
"It goes without saying that if rules were broken then there will be disciplinary action for all those involved," Mr Johnson said.
Later, when details of the Downing Street quiz emerged, Mr Johnson said he "certainly broke no rules". Number 10 said that the prime minister "briefly took part virtually" to thank staff for their work during the pandemic.
Two people pictured with Mr Johnson - wearing tinsel and a Santa hat - were members of his closer staff who had come in to help him with the technology, according to Downing Street.
Number 10 also insisted that no rules were broken when a photo emerged of the prime minister and his staff with bottles of wine and a cheeseboard in the Downing Street garden from 15 May. Mr Johnson said that the photo showed "people at work", which was allowed under the rules.