Sue Gray broke civil service rules when she was being recruited by Keir Starmer to become his chief of staff, the government has said, just days after she was cleared by a watchdog to take up the new job.
The former senior civil servant - who conducted the investigation into partygate - quit Whitehall in March.
On Friday she was given the go ahead by the Advisory Committee on Business Appointments (Acoba) watchdog to begin her role with the Labour leader in September.
Acoba - which is chaired by former Tory cabinet minister Eric Pickles- said there was “no evidence” Gray had done anything to favour Labour while she was a civil servant.
But in a written statement to parliament on Monday morning, Cabinet Office minister Jeremy Quinn said a separate inquiry concluded she broke the rules.
“This process, led by the civil service, found that the civil service code was prima facie broken as a result of the undeclared contact between Ms Gray and the Leader of the Opposition,” he said.
Quinn said Gray should have told ministers or the civil service when she first spoke to Starmer in October last year, four months before she resigned.
“The guidance on the Declaration and Management of Interests for Civil Servants, which is enshrined in departmental HR policies, sets out that individuals must declare all relevant outside interests to their line manager as soon as they arise,” he said.
Gray’s appointment by Starmer proved controversial among Tory MPs given that she led the official probe into Boris Johnson and rule-breaking during lockdown.
The former prime minister and his allies complained it showed she was biased against him at the time, even though she compiled her report before any contact with Labour.
Bob Kerslake, the former head of the civil service who died on Saturday, dismissed Johnson’s criticism of Gray at the time as “patently absurd”.
Gray’s report in May 2022 proved to be a bombshell. It detailed events at which officials drank so much they were sick, sang karaoke, became involved in altercations and abused security and cleaning staff at a time when millions of people across the country were unable to see friends and family.