Sue Gray investigated 16 events among which the police found multiple breaches of Covid rules during the pandemic.
Her report criticises the culture and leadership in Downing Street.
It includes photos of gatherings and the names of some of those who attended.
Here we look at some of the key details in the 48-page report.
For each event, Ms Gray summarises the Covid regulations that applied at the time, and describes what happened and who attended. Here's what she wrote about some of the gatherings:
* People were there for separate meetings in the No 10 garden where alcohol was drunk; the prime minister brought cheese and wine from his flat for an outdoor meeting lasting between 40 minutes and an hour
* Ms Gray says there is no reason to suggest this was anything other than a further work meeting
* Gatherings of more than two people in public were banned unless they were "essential for work purposes"
* The investigation was told this was aimed at "boosting staff morale following a challenging period"
* It was planned in advance by officials; an email sent to staff by the PM's principal private secretary Martin Reynolds read: "Please join us from 6pm and bring your own booze"
* Concerns were expressed about whether it was appropriate
* Mr Johnson attended for about 30 minutes to thank staff before returning to his office for a meeting
* Gatherings of more than two people in public were banned unless they were essential for work purposes
* A number of people gathered in the Cabinet Room for a pre-planned event with food, alcohol and soft drinks
* Cleo Watson, a No 10 special adviser, said she had been asked to organise it
* The prime minister and the chancellor had no advance knowledge of the event, which lasted about 30 minutes
* Gatherings of two or more people indoors and more than six outdoors were prohibited
* Staff gathered in the press office area of No 10 to mark the departure of Lee Cain, the PM's communications chief
* It was not planned and occurred at about the time that "Wine Time Friday" would normally take place
* The prime minister attended on his way to his Downing Street flat, gave a speech and had a drink
* After 6pm, a meeting was held in the PM's flat to discuss the handling of Mr Cain's departure and that of former No 10 adviser Dominic Cummings; the PM joined it at about 8pm
* Sue Gray says she was able to gather only limited information on this gathering before the Met started investigating
* Indoor gatherings of two or more people were banned unless they were "reasonably necessary" for work purposes
* Two separate leaving events took place in No 10, one for director of communications James Slack and one for another official
* Alcohol was consumed and speeches were made at both events, and the two groups eventually came together in the Downing Street garden
* Some people damaged a child's swing/slide in the garden
* Staff members stayed until the early hours, the last leaving at 4.20am
* Gatherings of two or more people indoors or more than six outdoors were prohibited
In total, nine photographs from inside Downing Street appear in the report. They show:
* Mr Johnson, Mr Sunak and Cabinet Secretary Simon Case gathered in the Cabinet Room for a birthday party thrown in the PM's honour in June 2020
* Mr Johnson drinking with staff at an event to mark the departure of officials on 13 November 2020
Ms Gray's critique focuses on the context at the time of the parties, and her conclusions as to how they were allowed to happen at the heart of government during the pandemic.
* The pandemic "had a seismic impact on every aspect of life in the country", and restrictions were put in place to control the spread of the virus and keep the most vulnerable safe
* Ministers and officials worked long hours "under difficult conditions in buildings that could not be easily adapted as Covid secure workplaces"
* But those challenges "applied to key and frontline workers across the country who were working under equally, if not more, demanding conditions"
* The report highlights the personal sacrifices everyone made, including those not able to see "loved ones in their last moments or care for vulnerable family and friends"
* Regardless of "initial intent", many of the events investigated were not "in line with Covid guidance at the time"
* Some attitudes and behaviours were "inconsistent" with that guidance
* It is clear there were breaches of the Covid regulations and guidance, given the Met Police fined 83 people
* Ms Gray's her initial report in January commented on "failures of leadership and judgment in No 10 and the Cabinet Office"
* Today she says many events were attended by government leaders and "should not have been allowed to happen"
* "The senior leadership at the centre, both political and official, must bear responsibility for this culture"; the public have "a right to expect the very highest standards of behaviour" in Downing Street and "clearly what happened fell well short of this"
* Some staff felt unable to raise concerns about behaviour they had witnessed at work
* There were "multiple examples of a lack of respect and poor treatment of security and cleaning staff", which was "unacceptable"
* "A clear and robust policy covering the consumption of alcohol in the workplace" has been recommended and implemented
* Ms Gray says disciplinary action "is for others to consider"; her report does not amount to a "disciplinary investigation"
* She says it is important to acknowledge that those in the most junior positions attended gatherings at which their seniors were present, or indeed organised them
* She hopes junior staff learn from this experience and that this will be taken into account when considering any action
* Ms Gray says she is "pleased progress is being made in addressing the issues I raised"
* There have been changes to the organisation and management of Downing Street and the Cabinet Office that aim for clearer lines of leadership and accountability
No one was identified in Ms Gray's initial report, which was released as the Met began investigating the events.
With the police force's investigation now concluded, Ms Gray has named some senior officials and ministers in her final report. They include:
* Prime Minister Boris Johnson
* Chancellor Rishi Sunak
* Cabinet Secretary Simon Case, the most senior civil servant
* Cleo Watson, a former No 10 special adviser
* Dominic Cummings, the PM's former chief adviser
* Lee Cain, the PM's former communications chief
* James Slack, another former director of communications
* Martin Reynolds, the PM's former principal private secretary
* Helen MacNamara, a former ethics chief at the Cabinet Office
* The 16 events took place over a dozen dates between May 2020 and April 2021, when Covid restrictions were in force
* They were held in 10 Downing Street or the Cabinet Office, apart from one in the Department for Education
* Of these events, 12 were investigated by the Metropolitan Police, who issued 126 fines to 83 people, including the prime minister
* Mr Johnson, his wife Carrie Johnson and Chancellor Rishi Sunak were all fined for attending a birthday party for the PM in the Cabinet Room on 19 June 2020
* Other events investigated by Ms Gray included the infamous bring-your-own-booze party in the Downing Street garden, attended by the PM on 20 May 2020
* She also looked into a leaving do for Mr Johnson's communications chief Lee Cain and a gathering in the No 10 flat, both on 13 November 2020
* The last events on the list were two parties held by Downing Street staff at No 10, the night before Prince Philip's funeral on 16 April 2021