Sue Gray, who investigated the “Partygate” scandal, is facing criticism for accepting a job as Sir Keir Starmer’s top aide.
Ministers believe that her appointment may have broken civil service rules.
Paymaster General Jeremy Quin argued that Ms Gray should have obtained approval to work for the opposition before leaving her post last week.
Tory MPs expressed anger at Sir Keir’s recruitment, with one describing it as a “rotting fish”.
They accused Labour of violating Whitehall’s impartiality by hiring a top civil servant. Allies of Boris Johnson argued that Ms Gray’s investigation into Partygate last year was a setup.
Despite criticism, Sir Keir defended his decision to hire Ms Gray, saying she is known for her integrity and delivery in government.
A top civil servant, Susan Acland-Hood, raised concerns in a video call that Ms Gray may have breached impartiality rules by holding secret meetings with Sir Keir.
As the most senior civil servant position in a government department, the role of permanent secretary requires impartiality.
Ms Gray was expected to inform the Advisory Committee on Business Appointments (Acoba) when she first discussed the chief of staff position.
An Acoba spokesperson stated that the process is confidential, and there is no timeline for the decision.
Ms Gray’s appointment comes after she produced a critical report into lockdown gatherings held in Downing Street while Boris Johnson was still Prime Minister.
Mr Johnson is facing another inquiry into whether he misled Parliament when he said Covid rules had not been broken in Downing Street.