Sir Keir Starmer is replacing the top civil servant in his Downing Street office.
Principal private secretary Nin Pandit runs the prime minister’s team at No 10 and has only been in the role for ten months.
She is the third senior staff member the prime minister has appointed and later let go, after chief of staff Sue Gray left in October and director of communications Matthew Doyle stood down in March.
Ms Pandit’s appointment had been announced by No 10 in the same press release that marked Ms Gray’s departure.
The BBC understands the prime minister had become concerned Ms Pandit was ineffective in the role, although No 10 contests this and allies of the prime minister said he retained trust and confidence in her.
It is expected Ms Pandit will move into a new role leading on policy delivery at No 10.
The BBC has contacted Ms Pandit for comment.
The prime minister’s principal private secretary (PPS) is one of the most prized jobs in the civil service and often a precursor to running major government departments or the service as a whole.
They act as the prime minister’s gatekeeper to the official government machine and “run their life” according to one former insider.
They control the information flow to the prime minister, shape their diary and are often the last person in the room when major decisions are made.
The PPS usually sits alongside the chief of staff just outside the prime minister’s office in No 10.
Before becoming PPS, Ms Pandit ran the No 10 policy unit during Rishi Sunak’s premiership. Earlier in her career she was chief of staff to the chief executive of NHS England.
After a series of Labour parliamentary rebellions before the summer break, many in government are also expecting Sir Keir to soon begin a reshuffle of his junior ministers.