Her inquiry follows the Metropolitan Police’s investigation into Covid lockdown breaches across Downing Street and Whitehall.
Boris Johnson, his wife Carrie and Chancellor Rishi Sunak were among the 83 people fined for attending events spanning eight separate days.
They all received one fixed penalty notice (FPN) for attending an event to celebrate the Prime Minister’s 56th birthday in June 2020. Indoor mixing was banned at the time of the gathering.
A total of 126 fines were issued by the force - with several people receiving multiple FPNs.
Now, the focus will shift to Ms Gray’s report which will be published after revelations of a so-called “secret” meeting between her and Mr Johnson.
Downing Street has admitted that it “instigated” the meeting but the Prime Minister would not be drawn on what was discussed.
But who is Sue Gray, what is her background and how did she end up in charge of the partygate inquiry? Here’s what you need to know.
Who is Sue Gray?
Once described as “deputy God”, Sue Gray is second permanent secretary at the Department for Levelling Up, Housing and Communities, and was previously director-general of propriety and ethics in the Cabinet Office from 2012 to 2018.
She is widely seen as a figure who would not pull any punches in an inquiry.
Ms Gray oversaw the Plebgate inquiry in 2012 after former chief whip Andrew Mitchell was accused of calling a policeman a “pleb” at the Downing Street gates.
She was once described as “deputy God” by then Labour MP Paul Flynn in a meeting of Parliament’s Public Administration Committee the same year.
Former Tory MP and Cabinet office minister Oliver Letwin is reported to have said of Ms Gray: “It took me precisely two years before I realised who it is that runs Britain. Our great United Kingdom is actually entirely run by a lady called Sue Gray, the head of ethics or something in the Cabinet Office. Unless she agrees, things just don’t happen.”
She is also part of the panel deciding on who will be next chair of the media regulator Ofcom.
Ms Gray was once described by BBC Newsnight’s then policy editor as “the most powerful person you’ve never heard of”.
Who is Sue Gray married to?
Sue Gray is married to country and western singer Bill Conlon.
The couple ran a pub in Northern Ireland in the 1980s as Ms Gray took a break from her civil servant role.
Mr Conlon’s music consists of traditional Irish ballads and has played European festivals with his band Emerald.
The two tend to live a private life away from the glare of the public spotlight, until the Downing Street party inquiry.
What happened with Simon Case?
Simon Case, who is the Cabinet Secretary and head of the home civil service, was appointed last December by the Prime Minister to lead the partygate inquiry.
However, reports soon emerged that he had attended a ‘Christmas drinks’ event with around 15 to 20 members of staff.
He resigned from the inquiry, with Ms Gray later replacing him.
After Mr Case was tasked with the investigation, the Prime Minister’s press secretary insisted that “due diligence had been followed” when asked if the Cabinet Secretary had attended the Downing Street party at the centre of the saga.
Cabinet Office minister Michael Ellis told the Commons that Mr Case “was not at any relevant gathering”.