Dominic Cummings’ call for “weirdos and misfits” to work at No 10 to help him transform the running of government is likely to be unlawful, experts are warning.

Boris Johnson’s chief aide is under fire on ageism grounds after saying he is targeting graduates – and for combative language described as “quite outrageous from an employment law perspective”.

The rambling 3,000-word blogpost has also provoked accusations of “hubris” from a former head of the civil service and cabinet fears that Mr Cummings risks “destabilising” Whitehall at a crucial time.

The head of Britain's biggest civil servants’ union said the comments implied he “wants to hire and fire at will”, revealing an anti-trade union mentality that would be “strenuously resisted”.

At one point, Mr Cummings wrote that “if you cut it you will be involved in things at the age of 21 that most people never see”.

Bizarre: Boris Johnson drives 'Brexit' digger through boxes Show all 8 1 /8 Bizarre: Boris Johnson drives 'Brexit' digger through boxes Bizarre: Boris Johnson drives 'Brexit' digger through boxes Conservative party leader Boris Johnson drives a JCB through a fake wall POOL/AFP via Getty Images Bizarre: Boris Johnson drives 'Brexit' digger through boxes Prime Minister Boris Johnson during a visit to JCB cab manufacturing centre in Uttoxeter, while on the General Election campaign trail. PA Photo. Picture date: Tuesday December 10, 2019. See PA story POLITICS Election. Photo credit should read: Stefan Rousseau/PA Wire Stefan Rousseau PA Bizarre: Boris Johnson drives 'Brexit' digger through boxes Britain's Prime Minister and Conservative party leader Boris Johnson drives a Union flag-themed JCB, with the words "Get Brexit Done" inside the digger bucket, through a fake wall emblazoned with the word "GRIDLOCK", during a general election campaign event at JCB construction company in Uttoxeter, Staffordshire, on December 10, 2019. - Britain will go to the polls on December 12, 2019 to vote in a pre-Christmas general election. (Photo by Ben STANSALL / POOL / AFP) (Photo by BEN STANSALL/POOL/AFP via Getty Images) BEN STANSALL AFP via Getty Bizarre: Boris Johnson drives 'Brexit' digger through boxes Prime Minister Boris Johnson sits in a cab of a JCB during a visit to its cab manufacturing centre in Uttoxeter, while on the General Election campaign trail. PA Photo. Picture date: Tuesday December 10, 2019. See PA story POLITICS Election. Photo credit should read: Stefan Rousseau/PA Wire Stefan Rousseau PA Bizarre: Boris Johnson drives 'Brexit' digger through boxes Britain's Prime Minister and Conservative leader Boris Johnson drives a Union flag-themed JCB, with the words "Get Brexit Done" inside the digger bucket, through a fake wall emblazoned with the word "GRIDLOCK", during a general election campaign event at JCB construction company in Uttoxeter, Britain, December 10, 2019. Ben Stansall/AFP/Pool via REUTERS POOL REUTERS Bizarre: Boris Johnson drives 'Brexit' digger through boxes A sign made for a visit by Prime Minister Boris Johnson to the JCB cab manufacturing centre in Uttoxeter, while on the General Election campaign trail. PA Photo. Picture date: Tuesday December 10, 2019. See PA story POLITICS Election. Photo credit should read: Stefan Rousseau/PA Wire Stefan Rousseau PA Bizarre: Boris Johnson drives 'Brexit' digger through boxes Britain's Prime Minister and Conservative party leader Boris Johnson drives a Union flag-themed JCB, with the words "Get Brexit Done" inside the digger bucket, through a fake wall emblazoned with the word "GRIDLOCK", during a general election campaign event at JCB construction company in Uttoxeter, Staffordshire, on December 10, 2019. - Britain will go to the polls on December 12, 2019 to vote in a pre-Christmas general election. (Photo by Ben STANSALL / POOL / AFP) (Photo by BEN STANSALL/POOL/AFP via Getty Images) BEN STANSALL AFP via Getty Bizarre: Boris Johnson drives 'Brexit' digger through boxes Britain's Prime Minister and Conservative party leader Boris Johnson reacts as he sits in the cab of a drives a Union flag-themed JCB, after driving through a fake wall emblazoned with the word "GRIDLOCK", during a general election campaign event at JCB construction company in Uttoxeter, Staffordshire, on December 10, 2019. - Britain will go to the polls on December 12, 2019 to vote in a pre-Christmas general election. (Photo by Ben STANSALL / POOL / AFP) (Photo by BEN STANSALL/POOL/AFP via Getty Images) BEN STANSALL AFP via Getty

“We want to hire some VERY clever young people either straight out of university or recently out with extreme curiosity and capacity for hard work,” he said.

But Philip Landau, a specialist employment lawyer, said: “It is not usually advisable to set out a specific age in an advertisement in case it can amount to discrimination. This is unless there is an occupational requirement to do so.”

He criticised the candid language, telling The Guardian: “A lot of employers these days are more keen to promote flexible working and a healthy work-life balance.

“Employers do need to be careful not to be discriminatory in their job advertisements where this may relate to one of your protected characteristics under the Equality Act (which includes but is not limited to age, disability, race, religion and sexual orientation).”

John Bowers QC, a leading employment barrister, recalled what he described as Mr Cummings “cavalier approach” last year when he sacked a young Treasury aide.

Sonia Khan was frogmarched by armed police from Downing Street after a confrontation with No 10’s chief strategist, which may have been unfair dismissal, Mr Bowers warned at the time.

On the latest blogpost, he said: “I am surprised the cabinet secretary allowed this advert to go out,” calling it “quite outrageous”.

Mr Cummings, the former Vote Leave chief, had protested that we “do not have the sort of expertise supporting the PM and ministers that is needed” – but argued that Mr Johnson, with an 80-seat majority, had “little need to worry about short-term unpopularity”.

Instead of “Oxbridge humanities graduates” he called for “true wild cards, artists, people who never went to university and fought their way out of an appalling hellhole, weirdos from William Gibson novels”.