Sue Gray has been appointed by Labour leader Sir Keir Starmer to serve as his chief of staff.
The senior civil servant became a household name during the partygate scandal, authoring the report which found “a failure of leadership and judgement” in Number 10 during Boris Johnson’s premiership.
Conservative MPs and former cabinet ministers have reacted furiously to the appointment.
A friend of former prime minister Boris Johnson said: “Keir Starmer appointing Sue Gray as his chief of staff reveals what many have suspected all along: partygate was a deliberate and manufactured plot to oust a Brexit-backing Conservative prime minister.
“The validity of the Sue Gray investigation and its findings is now completely destroyed.”
They added: “It leads to the inevitable conclusion that partygate was a Labour stitch-up.”
Ms Gray has left her role in the civil service ahead of taking up her new role, with her IT access and entry pass revoked.
It is not clear when she will take on the role, as the appointment will need to be checked by the watchdog which oversees new jobs for former ministers and senior civil servants – the Advisory Committee on Business Appointments (ACOBA).
According to the ACOBA website, the body provides advice to the prime minister about appointments.
It will be up to Rishi Sunak to make a final decision on agreeing to the appointment.
Some in the Conservative Party are unhappy that a person they believe contributed to Boris Johnson’s downfall has been tapped for a job by the opposition party.
Johnson-era cabinet ministers, including Nadie Dorries, Jacob Rees-Mogg and Andrea Leadsom all registered their disappointment.
Ms Dorries said it showed the partygate report was a “stitch-up”, Mr Rees-Mogg said “so much for an independent Civil Service”, and Ms Leadsom said “surely not?”
Labour MP and former minister Ben Bradshaw – currently a shadow minister – said Ms Gray was “just as strict” when Labour was in government.
Ms Gray has long had a fearsome reputation in Whitehall, serving as the key enforcer of ministerial behaviour while head of propriety and ethics in the Cabinet Office between 2012 and 2018.
The possibility she could be appointed to the Labour leader’s office was first reported by Sky News on Wednesday.
Ms Gray is the daughter of Irish immigrants and her son Liam Conlon is the chair of Labour’s Irish Society – a group formerly led by Starmer’s director of communications, Matthew Doyle.
Ms Gray’s appointment follows the dismissal of Sam White, who served as Sir Keir’s chief of staff for a year but was unpopular with some of the shadow cabinet.
Allies of the Labour leader had said that due to the relative inexperience of his team, a candidate who understands how to operate at the top level of government is essential.
Ms Gray’s appointment echoes Tony Blair’s recruitment of diplomat Jonathan Powell while Labour was in opposition in 1995.
Mr Powell later became Number 10’s first chief of staff and played a role in Northern Ireland peace talks before the Good Friday Agreement.
A Labour Party spokesperson said: “The Labour Party has offered Sue Gray the role of chief of staff to the leader of the opposition.
“We understand she hopes to accept the role subject to the normal procedures. Keir Starmer is delighted she is hoping to join our preparation for government and our mission to build a better Britain.”