Plans by Boris Johnson and his adviser Dominic Cummings for a “seismic” shake-up of the civil service have been branded “insulting” by a union representing Whitehall mandarins.

Civil servants could be made to take regular exams to prove they are up to their jobs as part of a government drive to create “the most dynamic state in the world”, according to one of the people behind the election-winning Conservative manifesto.

Rachel Wolf, who helped draw up the blueprint of Tory election pledges, told The Daily Telegraph that civil servants are “woefully unprepared” for sweeping reforms the prime minister is keen to push through.

She said the changes will end the current “merry-go-round” of officials regularly changing jobs, which mean that anyone staying in the same post for longer than 18 months is seen to have “stalled” in a culture that ensures “everyone rises to their position of incompetence”.

Mr Cummings is a long-standing critic of Whitehall and has in the past said that the principle of a permanent civil service was “an idea for the history books” and complained that “almost no one is ever fired” from senior posts due to involvement in failed projects.

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But the general secretary of the FDA union, which represents senior civil servants, said that the proposed reforms demonstrated “a lack of understanding” of modern Whitehall.

Dave Penman told the PoliticsHome website: “Tired old rhetoric of ‘civil servants being promoted to a level of incompetence’ is not only insulting, but demonstrates a lack of understanding of the modern realities of the civil service.

“All senior civil service jobs are externally advertised, meaning anyone promoted has not only competed successfully against their peers, but also with external candidates.

“Indeed many of the issues the civil service faces are of the government’s own creation. Churn in senior civil service roles is a result of a decade of pay stagnation, with movement between jobs the only route to a pay rise.”

Writing in The Daily Telegraph, Ms Wolf said reported plans for merging, creating or abolishing departments are just a “tiny fraction” of the changes set to be implemented after the UK’s scheduled departure from the EU at the end of this month.

Changes will be designed to ensure that civil servants are “reoriented to the public”, rather than “stakeholders”, she said. But she dismissed suggestions the civil service will be “politicised” under the reforms.

Many officials “cannot believe the prime minister and Dominic Cummings mean business”, said Ms Wolf, and “as a result, they seem woefully unprepared for what is coming”.

But Mr Penman said the proposed changes seemed less thoroughgoing than reforms introduced by former Tory minister Francis Maude during the 2010-15 coalition government.

“Whilst those advocating reform may like to paint the civil service as antiquated or resistant to change, the reality is somewhat different,” said the union boss.

“The UK civil service, recently ranked first in an international analysis of effectiveness, has had to constantly reform and adapt as each government sets out its new priorities.

“Indeed, the reforms being trailed are more modest than, say, the challenges the civil service faced in 2010. Whilst supporting the first coalition government since the Second World War, it had to manage cuts of 20 per cent in resources, deliver a radical policy agenda and at the same time institute a series of reforms under Francis Maude, transforming how government worked.”