Ministers who attack the civil service as “the blob” are guilty of using “dehumanising” language and “self-defeating cowardice”, the UK’s top mandarin has said.

Simon Case, the country’s most senior civil servant, said he had witnessed an “increased number of attacks” on the civil service that had “undoubtedly undermined the good functioning of government”.

The cabinet secretary said the last five years – spanning the premierships of Boris Johnson, Liz Truss and now Rishi Sunak – had seen a “deterioration in relations between officials and politicians”, although he said the situation had improved since Mr Sunak took office.

In recent years a number of ministers and former ministers have taken to blaming “the blob” – a byword for the civil service – for their frustrations over policy, most notably Brexit.

Asked about attacks on the establishment “blob” by current and recent ministers, Mr Case said: “Obviously I don’t agree with a characterisation which is insulting, dehumanising, totally unacceptable.

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“It would surprise me if current ministers were using this language, not least because if they were it would indicate something akin to self-defeating cowardice.”

He added: “Self-defeating because insulting the people who work for you, who are delivering public services on your behalf, advising you day in and day out… and cowardice because you know these people can’t answer back.”

Mr Case made the comments as he appeared in front of parliament’s public administration and constitutional affairs committee on Wednesday.

During the session, he referenced a recent incident in which the Tory chairman, Greg Hands, was forced to apologise to union bosses after a message was sent out by the party accusing civil servants of being part of an “activist blob”.

The email was sent to supporters under the name of Home Secretary Suella Braverman, claiming an “activist blob of left-wing lawyers, civil servants and the Labour Party” had prevented the government from stopping migrants on small boats coming to the UK.

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Sue Gray ‘broke civil service code’

Former cabinet minister Jacob Rees-Mogg also accused Mr Sunak of “surrendering” to the “blob” after he decided to scale back post-Brexit plans to scrap EU laws.

Elsewhere in the session, Mr Case denied suggestions that the departure of Sue Gray from the civil service to join the Labour Party had stoked concerns about its impartiality.

Mr Case said he believed some people had sought to “weaponise” the row amid broader attacks on the civil service.

Responding to questions from Tory MP David Jones, Mr Case said: “Actually, it has been particularly important that we assure ourselves and ministers that ministers are comfortable and happy that the impartiality of the civil service has been upheld. They are satisfied it has.

“Undoubtedly, there are people who have sought to weaponise this case.”

Ms Gray, who became a household name when she investigated the partygate scandal that engulfed Mr Johnson’s premiership, was approached by Labour leader Sir Keir Starmer about whether she would be interested in a role with the party while she was a civil servant.

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A subsequent investigation by the Cabinet Office found that Ms Gray – who will join Labour as Sir Keir’s chief of staff in September – broke the civil service code by discussing the job with the party leader and not declaring the contact.

However, the post-government appointments watchdog, the advisory committee on business appointments (Acoba), approved her appointment but said there had to be a six-month break between her quitting and her starting at Labour to avoid “a potential risk to the perceived impartiality of the civil service”.

Mr Case said that while there was “quite a lot of shock” when the news broke, he ultimately did not think Ms Gray’s move had undermined the impartiality of the civil service.