A rambling blogpost by Boris Johnson’s chief adviser, Dominic Cummings, calling for “weirdos and misfits with odd skills” to apply for jobs within No 10 may fall foul of employment law, experts have suggested.

One of the UK’s top employment lawyers described the post – which circumvented the usual Whitehall recruitment process and was peppered with the type of combative language Cummings has become known for – as “quite outrageous from an employment law perspective”.

“I am surprised the Cabinet secretary allowed this advert to go out,” said John Bowers QC, a leading employment barrister who also recalled what he described as Cummings’ “cavalier approach” last year when he sacked a young aide.

Bowers had said at the time that Sonia Khan, a Treasury media adviser who was escorted by armed police from Downing Street after a confrontation with No 10’s chief strategist, may have been unfairly dismissed from her job.

Philip Landau, a specialist employment lawyer at Landau Law Solicitors, identified potential issues with Cummings’ mentions of the likely age of candidates.

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

Landau 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.”

It was clear that this was not a conventional job advertisement, in terms of the candid language used for the role, said Landau.

“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).”

The head of Britain’s biggest civil servants’ union said that Cummings’ comment: “I’ll bin you within weeks if you don’t fit – don’t complain later because I made it clear now”, implied that he “wants to hire and fire at will” and revealed an anti-trade union mentality that would be “strenuously resisted”.

“The major problem for the civil service in the last decade has been under-investment, real-terms pay cuts and poor government policy,” Mark Serwotka, the general secretary of the Public and Commercial Services Union (PCS), said in a statement.

“Civil servants work tirelessly to make the machinery of government work for the public. However, when you shrink the civil service by over 18% since 2010, you are not going to be able to deliver the same level of service.”

A former head of the civil service, Lord Kerslake, cautioned Cummings against launching a “war” on the government service, warning that it cannot be changed overnight.

Kerslake told BBC Radio 4’s Today programme: “We need to wait to see the detail of this. But what I would say is of course the civil service should be open to challenge, to improvement and change – that’s part and parcel of how it stays a good civil service.

“What I would guard against is getting into a war with the civil service where they get given the blame, if you like, for anything that doesn’t quite go right – it works a lot better if you deliver change with the grain of the civil service.”