Keir Starmer has defended his controversial decision to offer Sue Gray a role as his next chief of staff, calling the senior civil servant a “really strong professional”. The Labour leader said on Friday he was delighted that Gray, who oversaw the internal inquiry into Downing Street parties during lockdown, had accepted his offer to become one of his top officials. Conservatives have voiced outrage about the proposed appointment, saying it tainted the inquiry she led which helped bring down Boris Johnson as prime minister. Meanwhile the government appointments watchdog could delay Gray’s start date by up to two years to minimise concerns over potential conflicts of interest. Starmer said on Friday however that Gray’s acceptance of the job showed the party was a serious government-in-waiting. He said: “ I want to ensure that we’re in a position to deliver for the whole of the United Kingdom and that’s why I’m so pleased that people of real quality, [who are] really respected, want to join the Labour team.” He added: “I hope we can change from this failure of the last 13 years to incoming Labour government, and in that I’m delighted that the really strong professional, respected individuals are now wanting to be part of that future.” Starmer was speaking after news of the planned appointment sparked a barrage of criticism from allies of Boris Johnson, who said it showed that the so-called Partygate inquiry was flawed from the start. They were not able to say which parts of the report, which catalogued a series of raucous parties held in Downing Street during lockdown, they disagreed with however. The Advisory Committee on Business Appointments (Acoba) will now decide the circumstances under which Gray can take up her new job, having already quit her job in the Cabinet Office. The committee’s rules say it can approve appointments with restrictions, including a ban on former civil servants lobbying government on behalf of their new employers, and having to wait for up to two years before taking the job. The rules do not say that Acoba can recommend blocking the appointment altogether, although they add that the final decision is for the prime minister. Rishi Sunak was reported on Friday to be concerned that Gray could use “privileged information” from her Downing Street days to help Starmer. The Times reported on Friday that it is likely Gray will serve out a “relatively short period of gardening leave” before starting in Starmer’s office. Lucy Powell, the shadow culture secretary, said the party would abide by the Acoba recommendations whatever they say, even though they are only advisory. Powell told the BBC’s Today programme: “Sue Gray and Keir Starmer have made absolutely clear they will abide by those recommendations, which is not always the case in these situations.” Labour said Starmer had offered the role to Gray after an extensive search – a decision that Labour officials said showed how serious the party’s leader is about preparing for government. Some have likened it to Tony Blair’s appointment of the senior diplomat Jonathan Powell as his chief of staff in 1995. Meanwhile, the Cabinet Office is reviewing whether Gray accepted the offer before informing the appointments watchdog. Many Conservatives are demanding to know when Labour first made contact with Gray, and whether they did so while her Partygate inquiry was ongoing. The Times reported that Starmer personally contacted Gray months ago after identifying her as the best person for the job. Powell said on Friday she did not know when the party first made contact with Gray or whether Starmer would eventually publish a timeline of how the job offer was made. • This article was amended on 5 March 2023. An earlier version misquoted Keir Starmer as saying “this failure of the last 30 years” when it should have been “13 years”.