Boris Johnson’s allies are showing a “fundamental misunderstanding” of the modern civil service, a union boss has said, amid warnings Number 10 is planning “seismic” changes.

Dave Penman, general secretary of the FDA, which represents senior civil servants, insisted those working in the civil service are not resistant to change and appointments are made on merit.

He also criticised a “decade of pay stagnation” as he spoke against “reform for reform’s sake”.

PCS general secretary Mark Serwotka also suggested some in Mr Johnson’s circle have an “anti-trade union mentality”, adding this will be “strenuously resisted”.

Their remarks came after Rachel Wolf, who helped draw up the blueprint of Tory election pledges, said civil servants could be made to take regular exams to prove they are up to their Whitehall jobs under “seismic” changes being planned by Number 10.

She also claimed that civil servants are “woefully unprepared” for sweeping reforms the PM is keen to push through.

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.

Ms Wolf said the changes, set to begin in the spring after the UK is due to leave the EU at the end of the month, will end the “merry-go-round” of officials changing jobs every 18 months.

But she dismissed suggestions the civil service will be “politicised” under the reforms, which the PM’s special adviser Dominic Cummings is likely to have a big influence over.

Mr Penman said talk of “swingeing” civil service reform is “nothing new and somewhat expected” as each new government sets out its priorities.

He added: “Indeed, the reforms trailed in the Telegraph are more modest than the challenges that faced civil servants in 2010.

“Dealing with a series of reforms led by Francis Maude, the civil service managed to support the first coalition government since the Second World War and deliver its radical policy agenda, all while absorbing 20% cuts to resources.

“While painting the civil service as resistant to change might make a good headline, the reality is quite different and the idea that civil servants are rising ‘to their position of incompetence’ is so wide of the mark it demonstrates a fundamental misunderstanding of the realities of the modern civil service.

“All senior civil service roles are externally advertised and appointed on merit.

“The so-called ‘merry-go-round’ that Wolf decries is entirely of the Government’s own creation, with a decade of pay stagnation and the removal of pay progression leaving the movement between jobs as the only route to a pay rise.

“If the Prime Minister really does want to deliver 50 million new GP appointments, new train lines, or better bus services, we need to see a much clearer plan on how these reforms will lead to the transformation promised. Otherwise, we could be left with just another round of reform for reform’s sake.”

For the PCS, Mr Serwotka also said: “The major problem for the civil service in the last decade has been under investment, real terms pay cuts and poor government policy.

“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.

“Comments by Dominic Cummings that imply he wants to hire and fire at will reveal an anti-trade union mentality and will be strenuously resisted by PCS.”

Ms Wolf had said Mr Johnson wanted to run “the most dynamic state in the world”.

She said one of the biggest changes is likely to be in the area of Whitehall recruitment and training.

Ms Wolf stated that anyone staying in the same job for longer than 18 months is currently seen to have “stalled” in a culture that ensures “everyone rises to their position of incompetence”.

She also predicts that civil servants will be “reoriented to the public”, rather than “stakeholders”.

Many officials “cannot believe the Prime Minister and Dominic Cummings mean business”, she writes, and “as a result, they seem woefully unprepared for what is coming”.