The head of the civil service, Simon Case, has attempted to block former senior official Sue Gray from working with Labour as Keir Starmer’s chief of staff until after the next general election, the Guardian has been told. Whitehall sources said that Case, as Gray’s former civil service boss in the Cabinet Office, had “pushed for” the government’s appointments watchdog to delay her starting the new job for the maximum two years. Starmer’s choice for his new chief of staff caused shock at Westminster when it was unveiled last month as Gray had led the civil service inquiry into the Partygate scandal and ran the Cabinet Office’s propriety and ethics team until 2018. However, the role is still subject to approval by the Advisory Committee on Business Appointments (Acoba), which can suggest cooling-off periods of between three months and two years for senior ex-ministers or civil servants who want to take up new jobs, to minimise concerns over potential conflicts of interest. The Sun has reported that, according to government sources, Gray faces a ban from working with Labour until after the next election, expected next year, meaning the former Whitehall enforcer would be unable to oversee the party’s transition to power should it win. Some ministers and Tory MPs are concerned about her previous sensitive role at the Cabinet Office. Sources told the Guardian that the cabinet secretary, who had to sign off Gray’s Acoba application, was pushing for the longest delay possible. He is understood to have personally clashed with Gray both during her Partygate investigation and afterwards, when he blocked her promotion to a more senior role, effectively limiting her career prospects within the civil service. One government insider said: “Case has pushed for Acoba to postpone her appointment by a minimum of a year, but he actually wants the longest delay possible. They had very different ideas on how the civil service should do its job.” In her final months in the civil service, Gray worked for both the levelling up department and the Cabinet Office. Oliver Dowden, Gray’s ministerial boss at the Cabinet Office, is understood to want Acoba to delay the appointment for “as long as possible”. Downing Street is said to be keen to put Starmer, who has made a virtue of sticking to the rules, in the awkward position of having to decide whether to ignore the watchdog’s advice. However, government sources suggested that Michael Gove, Gray’s ministerial boss at the department for levelling up, was “fairly agnostic” about how long she should have to wait. When Gray resigned after news of her new role with Labour leaked before it had been flagged to Acoba, the Cabinet Office announced that it would review the circumstances under which she had left. One government source told the Guardian the review had found that she may have breached the civil service code by discussing the chief of staff role with the leader of the opposition. However, other insiders suggested that the review had not yet concluded. Throughout, Starmer has defended his decision to offer Gray a role as his next chief of staff, calling the senior civil servant a “really strong professional”. A Cabinet Office spokesperson said: “We would not comment on private individuals. Acoba has confirmed that it is examining this case and will make its independent recommendations in due course.” Under the Acoba advice process, which is independent from government, department chiefs would have had to sign off Gray’s application to the watchdog, which then considers the information provided. Acoba will then come to a majority view on the proposed appointment and any conditions such as a delayed start date, before providing Gray with its provisional advice, which she is permitted to query. It is not until this process is complete that Acoba makes its recommendation to Rishi Sunak, who will make a final ruling, but does not have the power to block an appointment. He is also likely to take advice from Case at this stage. However, Labour sources indicated that neither Gray nor the party had yet been given the Acoba advice. Boris Johnson’s allies were furious when it emerged that Starmer intended to hire Gray, who led the initial inquiry into Partygate, launching an all-out effort to undermine the parliamentary inquiry into whether he had lied to MPs over the scandal. However, she received backing from other Tories including former Cabinet Office minister Francis Maude, who said he never had the “slightest reason” to question either her integrity or her political impartiality, and that she was not the first civil servant to move to a political role and would not be the last. Starmer was said to have been seeking a chief of staff in the mould of Tony Blair’s recruitment of the diplomat Jonathan Powell to help prepare a team and shadow cabinet that are relatively inexperienced when it comes to government. Government sources said they would refute the suggestion that Gray’s progress in the civil service was blocked by Case, pointing out that permanent secretary job appointment processes are overseen by the first civil service commissioner. The source said no action was taken against Case as part of the Partygate investigation, from which he recused himself, and there would have been no reason for any tensions between the pair.