In the 1980s, Sue Gray briefly paused her stellar civil service career to buy a pub in “bandit country” in Northern Ireland during The Troubles. If Boris Johnson needed evidence that his inquisitor-in-chief is no pushover, then being landlady of a pub in Newry close to the Irish border shows she is no soft touch.

Ms Gray has the task of investigating a series of alleged lockdown parties held in Downing Street and across government during the Covid-19 pandemic. 

The Prime Minister has been questioned by Ms Gray over the “Partygate” allegations he and other MPs face and The Telegraph understands that he has shared all he knows with the civil servant.

Dominic Cummings is also being interviewed by Ms Gray as part of her investigation. Mr Johnson’s former top aide, who has been critical of the Prime Minister since leaving No10, has previously claimed on his blog that he and other witnesses were prepared to swear under oath that Mr Johnson had “lied to Parliament about parties”.

Ahead of the report being published as early as this week, Downing Street is already planning its response – with a promise to clean up the culture of drinking and rule-breaking it seems to have adopted, as well as the potential sacking of senior members.

Ominously for the Prime Minister, an MP once described her as “deputy god”, while Sir Oliver Letwin admitted when in Cabinet that it had taken him “precisely two years before I realised who it is that runs Britain”, adding: “Our great United Kingdom is actually entirely run by a lady called Sue Gray. Nothing moves in Whitehall unless Sue says so.”

She does not suffer fools. For six years, she was director-general, propriety and ethics, in the Cabinet Office. 

She ran inquiries into “plebgate” – in which Andrew Mitchell, the former chief whip, was accused of calling a police officer a “pleb” – and into allegations that Damian Green had used his parliamentary computer to access pornography. Mr Green, then deputy prime minister, was forced to resign.

Now aged 63, Ms Gray has held senior civil service posts under a number of prime ministers, both Labour and Conservative, and has pulled off the trick of keeping politicians in the dark about her own political leanings. 

She remains on friendly terms with Alastair Campbell, Tony Blair’s former director of communications and strategy, and the pair remain in touch. 

“She is a very, very strong woman,” Mr Campbell said on January 11. “She really cares about what goes on in Government. I would not have a clue about her politics … but she is somebody whose judgement I have valued.”

In one volume of Mr Campbell’s diary, he references a number of meetings with her after leaving Downing Street – not least because one of Ms Gray’s chief roles was to vet political memoirs, to ensure no state secrets were given away or civil servants unfairly maligned.