The senior civil servant is expected to formally request to become a central member of Sir Keir’s top team by submitting her application to the advisory committee on business appointments (Acoba).
It comes as senior Tories continued to attack the decision by Ms Gray to take up the offer, due to her position investigating Boris Johnson’s role in the partygate scandal.
Former Tory party chair Jake Berry claimed her decision to join Labour could be viewed as the “biggest stitch up since the Bayeux Tapestry” by the public, adding Labour and Ms Gray had made a “mistake”.
“It’s this appearance of bias,” he told GB News.
And he added: “In all my dealings with Sue Gray I’ve always found her completely fair and impartial.
“But I think it plays into this biggest idea, that the British people voted for Brexit, and I was in Government and I saw first hand civil servants at every turn try to frustrate Brexit.
“The British people then voted for Boris Johnson to actually deliver the Brexit they had voted for a couple of years earlier, and then it feels like because of this appointment of Sue Gray we then saw the civil rice try to frustrate and remove Boris Johnson from office.”
Mr Johnson’s allies have repeatedly claimed that Ms Gray’s decision to join Sir Keir’s office shows the former prime minister removal from office was a “stitch up” and a “connivance”.
But Labour has insisted that the approach to hire a senior civil servant of the stature of Ms Gray shows how seriously the party is taking the prospect of being in government, a shadow minister has said.
Asked on Sky News if the appointment reflects badly on Labour, shadow work and pensions secretary Jonathan Ashworth said: “Sue Gray is a woman of exceptional talent, abilities, incredibly high calibre, and in the same way in which senior business people are now supporting the Labour Party, the same way we have a new crop of candidates of high calibre, I think it shows how seriously people are taking the prospect of Keir Starmer as prime minister and a Labour government.”
Ms Gray is already the subject of an informal probe by the propriety and ethics team that sits within the Cabinet Office as to whether she had broken the civil service code by failing to declare her conversations with Labour at the earliest opportunity.