Internal inquiries rarely force the resignation of a sitting prime minister, but the veteran civil servant conducting the one into Downing Street parties has previously helped end the careers of several cabinet ministers. And with all eyes in Westminster now on Sue Gray’s inquiry, her previous investigations offer clues to how she goes about her business – and they will not give Boris Johnson much comfort. Among the scalps claimed by Gray are those of Damian Green, Liam Fox, Andrew Mitchell and Hazel Blears. Her inquiry into Green showed her to be an uncompromising operator. This time she is not investigating a minister at the centre of a high-profile scandal and then passing on her findings to the cabinet secretary and the prime minister. At the behest of Johnson, she is establishing facts that will later be examined by the public and Tory MPs as they decide the future of the government. Gray has returned to work with the propriety and ethics team in the Cabinet Office – the unit set up to examine the integrity of ministers and officials – that she left in 2018. Previously, in 2017, she was invited to investigate Damian Green, the first secretary of state and de facto deputy to Theresa May. Gray examined two allegations that Green had broken the ministerial code. One was an allegation of inappropriate behaviour – an unwanted sexual advance – towards the journalist Kate Maltby, followed by an inappropriate text message. A second claim, that during a 2008 inquiry into leaks pornographic material was found on Green’s parliamentary computer, resulted in Green’s arrest. Maltby told the Cabinet Office inquiry that the MP touched her knee and appeared to solicit sex from her in exchange for political mentoring. After interviewing both parties, Gray released a damning 500-word summary of the inquiry, which examined the “competing and contradictory accounts” of Green and Maltby. It concluded: “It was not possible to reach a definitive conclusion on the appropriateness of Mr Green’s behaviour with Kate Maltby in early 2015, though the investigation found Ms Maltby’s account to be plausible.” Sources have told the Guardian that Gray insisted the word “plausible” should be included in the report – a phrase that was seized upon by Green’s critics. The findings concluded Green had also released two misleading statements, which had suggested he was not aware that indecent material was found on parliamentary computers in his office, even though the Metropolitan police had previously informed him of the existence of this material. Green, the former first secretary of state and widely seen as May’s closest friend in government, has said he accepted his dismissal because he was misleading in a statement about a police investigation into pornography found on his office computer. Gray’s report into Green, and the summary of the report, were handed to the then cabinet secretary Sir Jeremy Heywood, who shared them with the PM’s independent adviser on ministerial interests, who was then Sir Alex Allan. It remains unclear whether Simon Case, the cabinet secretary, will have any role in the current inquiry into drinks parties. He was initially appointed to lead the inquiry but was forced to step down after it emerged that his office had held one itself during lockdown. Critics have called for Johnson to recuse himself of his role overseeing the drinks party inquiry. If Gray suggests that there may have been a breach of the ministerial code by Johnson, he will then have to rule on whether to refer himself to Lord Geidt, the current independent adviser on ministerial interests. Gray, 64, is married to Bill Conlon, a County Down-based country and western singer, and left her job in propriety and ethics to take up a senior role in the Northern Irish civil service. She returned to England in April last year as second permanent secretary in the Cabinet Office with responsibility for the union, then moved with Michael Gove to his new Department for Levelling Up after the reshuffle last September.