Sir Keir Starmer was wooing Sue Gray to become his chief of staff for at least four months before announcing her appointment, Sky News has discovered.
The Labour leader has repeatedly refused to disclose when the first contact between the pair took place.
But Sky News understands his first discussion with the top civil servant came last October.
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Sir Keir’s office is understood to have initiated contact with her, rather than the other way around.
After this, he and his team had multiple conversations with Ms Gray, who was then a permanent secretary working on the constitution and the union of the UK.
The negotiation remained secret for months and the appointment was only confirmed in March, the day after Sky News broke the news of talks between her and Sir Keir.
This means the first communication between them came after the publication of her report into Boris Johnson, which was released in May 2022.
However, it was during attempts by MPs to secure information about her probe and Mr Johnson’s activities.
From October last year to March 2023, the Privileges Committee was working on its investigation into whether Mr Johnson lied to parliament, and asking for evidence from the government.
At the time, it was thought some of that evidence could have come from the work conducted by Ms Gray carried out earlier in the year, and it is unclear what she knew.
But the chair of the committee, Harriet Harman, later confirmed they relied on their own material for their inquiry, rather than Ms Gray’s report.
The contact between Sir Keir and Ms Gray came in one of the most turbulent months in British politics, which saw economic turmoil after Liz Truss’s mini-budget and saw her replaced by her former rival Rishi Sunak.
Labour is now waiting for a decision from the Advisory Committee on Business Appointments (ACOBA) – which has the power to delay appointments but not block them – to find out when she can start work.
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On Tuesday, it emerged that Ms Gray had not co-operated with a separate probe led by the Cabinet Office.
A larger report into her resignation had been expected by Whitehall watchers, but it is understood cabinet secretary Simon Case withheld material amid concerns about the sensitivity of the probe.
It also emerged on Wednesday that the First Division Association (FDA) union, led by general secretary Dave Penman, was providing support to Ms Gray during these inquiries, with no Labour involvement.
Labour and the Cabinet Office declined to comment.