The Prime Minister has appointed Simon Case as Cabinet Secretary and head of the Civil Service, the Cabinet Office has confirmed.

The 41-year-old will succeed Sir Mark Sedwill, who announced in June that he would be standing down amid rumours of a rift in 10 Downing Street.

Mr Case, a former private secretary to the Duke of Cambridge, takes on the job as the country’s top civil servant as the shake-up of Whitehall under Boris Johnson and adviser Dominic Cummings continues.

Currently the permanent secretary in Number 10, his new role will put him in charge of overseeing the day-to-day running of the Government and joining it up with the Prime Minister’s policy priorities.

Simon's years of experience at the heart of government and working for the royal household make him ideally suited for this crucial role

His appointment follows five departures of senior civil servants this year alone, under the so-called “hard rain” overhaul reportedly orchestrated by Mr Cummings, Mr Johnson’s de facto chief of staff.

The Prime Minister said: “Simon will make a fantastic Cabinet Secretary and head of the Civil Service.

“His years of experience at the heart of government and working for the royal household make him ideally suited for this crucial role.

“I would also like to thank Mark Sedwill for his outstanding service to the Government and the country as a whole.

“After serving for decades with great distinction, I believe he has earned the gratitude of the nation.”

Mr Case spent almost two years working as William’s right-hand man before temporarily moving to Number 10 earlier this year to assist with the coronavirus response.

His promotion to Cabinet Secretary comes after Sir Mark announced his departure from the role amid reports of clashes with Mr Cummings.

Mr Case said the appointment was “an honour” and praised his predecessor for the “kindness and support” shown to him during his career.

“Over these few months of working on the Covid response, I have seen how much hard work is being done by the civil service to support the government and our country through unprecedented times,” he said.

“It is a privilege to come into this role to lead a service that is working day in, day out to deliver for people right across the country.”

Lord (Gus) O’Donnell, who held the role between 2005 and 2011, said Mr Case, the youngest Cabinet Secretary in several decades, would have to pour his efforts into mending Number 10’s relationship with the civil service after a number of permanent secretaries were ousted, while ministers have been able to keep their jobs.

Apart from Sir Mark’s exit, three permanent secretaries have resigned this year, the Home Office’s Philip Rutnam, the Foreign and Commonwealth Office’s Simon McDonald, who leaves his position on Tuesday and Richard Heaton from the Ministry of Justice.

And last week, Jonathan Slater was removed from the post of permanent secretary at the Department for Education following the controversy surrounding this year’s A-levels and GCSEs, while Education Secretary Gavin Williamson, who came under pressure to resign following the exams fiasco, held on to his Cabinet job.

Speaking to BBC Radio 4’s Today programme, Lord O’Donnell said: “It is a surprise to have someone so young but I congratulate him and he has a lot of experience inside Number 10.

“He’s had lots of different jobs so I’m sure he will grow into the role, and I think the challenge for him is to restore trust in Government and restore the Civil Service’s trust, that’s what he’s got to get.

“At the moment we’ve got ministers blaming civil servants for everything, talking about reform without explaining why they want that reform and what’s going wrong at a ministerial level.

“There’s a big task for him but he has the trust of the Prime Minister, that’s really important, and now he needs to get the trust across the Civil Service and his fellow permanent secretaries.”

Apart from his role with the Duke of Cambridge, Mr Case’s career has included helping deliver the 2012 London Olympics, a tenure as private secretary to former prime minister David Cameron, and working on the Irish border issue created by Brexit.

The Times reported that the Cambridge graduate did not initially seek the top job but was asked to make a formal application by Downing Street.

According to Times Radio, the Prime Minister phoned William personally last week to “ask if he could pinch his man to be the new Cabinet Secretary”, with Mr Case, who joined the civil service in 2006, initially set to return to his duties at Kensington Palace.

The duke is said to have agreed given the tough circumstances faced by the country after the Covid-19 pandemic, with the station reporting that Mr Johnson and William have a good relationship dating back to the failed bid 10 years ago for England to host the 2018 World Cup.

Mr Case will formally take up the post on September 9, when Sir Mark stands down.