The UK’s top civil servant Simon Case has announced he will step down at the end of the year on health grounds.

Cabinet Secretary Mr Case, who has been undergoing treatment for a neurological condition, told colleagues “whilst the spirit remains willing, the body is not”.

There have been reports of tensions at the heart of No 10 between Mr Case and Sir Keir Starmer’s chief of staff Sue Gray, but the Cabinet Secretary said: “It is a shame that I feel I have to spell this out, but my decision is solely to do with my health and nothing to do with anything else.”

Mr Case returned to work in January after two months off sick and he said he had been “undergoing medical treatment for a neurological condition over the last 18 months”.

A recruitment process for the next cabinet secretary has begun.

In a sign of the turbulence in British politics, Mr Case said: “It has been an honour to serve two sovereigns, four prime ministers and over 120 Cabinet ministers in this role.

“There have been far more ups than downs along the way and by far the greatest highlight has been the privilege of working with so many remarkable public servants, across the length and breadth of our country, in our overseas posts and with counterparts from our close allies and partners around the world.”

Sir Keir Starmer thanked Mr Case for his work and said he had been “very fortunate to be able to draw on Simon’s advice in my first weeks and months as Prime Minister”.

“It is right that Simon takes time now to focus on his health, and he should know that he does so with the well wishes of this Government and all those who he has served during a truly remarkable career,” Sir Keir added.

Mr Case was appointed Cabinet Secretary by Boris Johnson in September 2020.

Evidence at the UK Covid-19 Inquiry revealed he was often exasperated by the administration.

He said he found Mr Johnson’s style of working “very frustrating” and described his day-to-day administration as “dysfunctional”.

In July 2020, before he became Cabinet Secretary, Mr Case said: “I’ve never seen a bunch of people less well-equipped to run a country.”

He also described Mr Johnson and his inner circle as “basically feral” and suggested the then-prime minister’s wife Carrie was “the real person in charge” in No 10.

Mr Case had previously worked as private secretary to the then Duke of Cambridge, the current Prince of Wales.