Boris Johnson is understood to be personally taking the blame for recent problems and senior Downing Street figures believe that criticism of Mr Case and other key aides is misplaced.

However, other government sources argued there was an atmosphere of rudderlessness within his administration, with some claiming Mr Case was partly responsible for key decisions not being forced through or delivered on.

One government insider said Mr Case was seen as “Dom’s choice” for the top Civil Service job, suggesting he had become tainted by association with Dominic Cummings, the former aide who is deeply out of favour with Mr Johnson and his allies.

Others in Whitehall claimed he has struggled to command the respect of some of the permanent secretaries over whom he presides. 

One civil servant said Mr Case did not “arbitrate disputes” between departments or “help set the direction” of the Civil Service in the same way his predecessors had. As a result, a higher proportion of administrative issues had to be escalated to ministers, it was claimed.

However, that narrative was fiercely challenged by a series of senior supporters. 

One government source said of him: “He’s incredibly intelligent and capable, and he grips every issue. He’s in all the important PM meetings. 

“He’s been integral to the response to the pandemic and other difficult issues. He’s a really important part of the team and impresses everyone.”

A former colleague who worked with Mr Case in Whitehall previously said that Cabinet secretaries tended to be either “fixers”, with a focus on the Prime Minister of the day, or “systems leaders” with a focus on Whitehall, adding that Mr Case fell into the former category.

“Simon has always been very good at fixing things for PMs and making himself useful to PMs,” the source said. Mr Case was previously principal private secretary to David Cameron and Theresa May.

The source argued that Mr Case has nonetheless also had “notable successes” championing the Civil Service, including winning extensions to the terms of several current permanent secretaries.

A Cabinet Office source meanwhile said that Mr Johnson “promoted Simon to Cabinet Secretary, which is a clear reflection of the confidence the PM has in his abilities”.

The source added that Mr Case “drives the Government response to all the biggest challenges we face on a daily basis”. 

The insider also pointed out that Jeremy Heywood, a predecessor as cabinet secretary, had also not run a department before taking on the role and “people did not question his competence”. 

Difficult week for Boris Johnson

The record low ratings for Mr Johnson and his administration came in the middle of a difficult week, following a backbench Tory revolt over social care and questions about whether the Prime Minister was okay in the wake of an unorthodox speech to business leaders.

Tory concerns over his performance have gathered momentum, with MPs – including even a party whip – now convinced that letters of no confidence have been submitted.