Ms Gray has interviewed Prime Minister Boris Johnson about what he knows regarding a string of party claims at No.10 Downing Street as part of the inquiry.
But what will the investigation look at and when will the report be published?
Here’s what you need to know.
What is the inquiry?
Sue Gray, a senior civil servant, has been tasked with investigating alleged breaches of lockdown rules within the Government.
Prime Minister Boris Johnson admitted to the House of Commons that he attended the gathering in May 2020, which 100 people were invited to and roughly 30 attended.
Speaking at PMQs on Wednesday 12 January, he offered a "heartfelt apology", but said he had believed it to be a work event.
At the time, people in England were only allowed to meet up with one person they did not live with outside.
Downing Street staff are also said to have held two parties the night before Prince Philip’s funeral in April 2021 - at a time when Covid restrictions banned indoor mixing.
The events, first reported by The Telegraph, took place on 16 April and went on until the early hours.
The Prime Minister was not at either party, but it raises further questions about a culture of rule-breaking at Downing Street.
Mr Johnson has refused to be drawn on the details of his involvement in parties at No 10 and has asked people to wait until the findings of the investigation.
What will it be looking into?
Ms Gray is thought to be examining emails, calendar invites, phone messages and eyewitness accounts.
However, some reports have suggested she may also have access to CCTV and police reports.
It is also believed that during her investigation, Ms Gray was given photographs of alleged gatherings, one of which reportedly shows the Prime Minister beside bottles of wine.
The inquiry is likely to lay out a timeline for each of the events, with Ms Gray expected to establish who organised them and how, who attended, and who knew about the parties.
When will the report be released?
Sue Gray is expected to present the results to the government on Thursday (27 January) at the earliest, but it is not yet confirmed when it will definitely be released.
It had originally been expected that the report could be presented on Wednesday (26 January) and published later that day, however this did not materialise.
Reports say that Ms Gray’s investigation is likely to be published no later than Monday (31 January).
The Government’s ministerial code adviser, Lord Geidt, is understood not to be involved in the process at the moment, but if Ms Gray’s findings do suggest that a minister – including Mr Johnson – has broken the ministerial code, then Lord Geidt may have to open a new investigation.
The Metropolitan Police’s special inquiry team is also leading an investigation into alleged rule-breaking Downing Street parties, but Sue Gray’s report is still set to be presented this week.
What could the report mean for Boris Johnson?
If the findings of Sue Gray’s report are damning, Tory MPs are expected to submit no confidence letters, with many said to be waiting for the outcome of the inquiry before deciding whether they back Mr Johnson’s leadership.
The threshold to trigger a no confidence vote in the PM will be reached if 54 letters are submitted.
If this figure is reached and Mr Johnson fails to receive the backing of 181 Tories, then a leadership contest will be announced.
With the Metropolitan Police now launching their own investigation into the alleged gatherings, those involved - including the Prime Minister - could be served fines in line with the punishment for the Covid rules in place at the time.
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