A former cabinet secretary has said his latest reported successor will need to regain the trust of the Civil Service after a host of high-profile departmental sackings and fallings-out in recent months.

Simon Case, a former private secretary to the Duke of Cambridge, is reportedly set to be named the UK’s new Cabinet Secretary as a shake-up of the Civil Service under Boris Johnson and adviser Dominic Cummings continues.

The rumour was further fuelled when Mr Cummings, who has become known for his informal work attire, was spotted arriving in Downing Street wearing a suit.

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

Lord (Gus) O’Donnell, who held the role between 2005 and 2011, said Mr Case 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.

The 41-year-old’s reported appointment follows five departures of senior civil servants this year alone, under the so-called “hard rain” overhaul reportedly orchestrated by Mr Cummings, the Prime Minister’s de facto chief of staff.

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.”

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 expected promotion to Cabinet Secretary comes after Sir Mark Sedwill announced that he was stepping down from the role amid reports of clashes with Mr Cummings.

The Financial Times reported that, in June, Mr Cummings told aides a “hard rain is coming” and promised to reform an “incoherent” Cabinet Office, making a smaller and more elite centre of government and overhauling Whitehall culture.

Apart from Mr Sedwill’s departure, three permanent secretaries have resigned this year – the Home Office’s Philip Rutnam, the Foreign and Commonwealth Office’s Simon McDonald, 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.

Civil Service leaders have complained that the departures represent an erosion of trust between the Government and civil servants.

A doctor of philosophy, Cambridge-educated Mr Case joined the Civil Service in 2006 and his relative lack of experience is reported to have made him a surprise choice to replace Sir Mark.

But Lord O’Donnell said he did not think Mr Case’s lack of experience in leading a government department would be a disadvantage, given that Lord Jeremy Heywood – one of his most prominent predecessors – had not been a permanent secretary either.

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.

Mr Case will be the youngest Cabinet Secretary since 1916 and is younger than all permanent secretaries bar one, The Times reported, adding that he did not initially seek the 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 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.

A Cabinet Office spokesman said: “An official announcement on the new Cabinet Secretary will be made on Tuesday 1 September.”