Boris Johnson’s plans to shake-up Whitehall risk being a wasteful “distraction” from the delivery of key public services, a union representing senior civil servants has warned.

And a former Europe adviser to Theresa May has warned that the government and civil service are not “match fit” for the negotiations with Brussels on a future trade relationship which will take up much of the coming year.

Bolstered by his 80-seat majority from Thursday’s general election, the prime minister is understood to be planning a “revolutionary” overhaul of the machinery of government, driven by his close adviser Dominic Cummings.

Stephen Barclay’s Department for Exiting the EU (Dexeu) will be shut down after the UK leaves the European Union on 31 January.

And wider changes under consideration include the merger of the departments for business and International trade and the incorporation of the Department for International Development into the Foreign Office, as well as the creation of a new department for borders and immigration outside the Home Office.

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The Sunday Telegraph reported that Mr Johnson was discussing proposals set out by Mr Cummings – who clashed with the civil service as a special adviser to Michael Gove in the Department for Education – to abolish the role of permanent secretaries running individual departments and create a strong “office of the prime minister” to exert greater control over Whitehall.

Changes under consideration are thought to include making it easier to hire and fire civil servants and to replace independent career mandarins with outside “experts”. Mr Cummings complained in a 2014 lecture that “almost no one is ever fired” in the civil service.

Mr Johnson is thought likely to conduct a limited reshuffle of his government on Monday to fill gaps left by ex-culture secretary Nicky Morgan, who stood down as an MP, as well as former Wales secretary Alun Cairns, who quit the government before the election, and environment minister Zac Goldsmith, who lost his Richmond Park seat on Thursday. Speculation was rife that Ms Morgan’s seat at the cabinet could be filled by Penny Mordaunt, Anne-Marie Trevelyan or the former culture secretary and prominent critic of the BBC John Whittingdale.

A more wide-ranging clearcut of the cabinet is expected in February, after EU withdrawal has taken place, and could cost the jobs of eurosceptics like Jacob Rees-Mogg, who were brought into government to “get Brexit done”.

An unnamed “senior figure” told The Sunday Times: “It will be pretty big. It will be finding the people who can do the jobs and not worry about media and short-term things. We’re drawing up a very detailed and very revolutionary plan and then we are going to implement it.”

Dave Penman, general secretary of the FDA, has said he is concerned over the reshuffle (Getty) (Photoshot)

Dave Penman, the general secretary of the FDA union, which represents senior civil servants, voiced caution over reforms. “Transformative government is delivered through policy, not reorganisation,” said Mr Penman.

“Whilst the temptation from every new government is to demonstrate their reforming zeal by reorganising Whitehall, this can often be a distraction and waste both time and money as civil servants merge or split departments, rather than simply getting on with the job in hand.

“Government departments, many of whom employ thousands of staff, are all separate employers with their own pay and grading structures. Putting these together alongside IT and back office functions takes time and resource.

“It’s never a short-term fix and only works if it’s supported by a clear policy focus and is embedded for the longer term.

“Civil servants are there to deliver the new government’s policy objectives and the prime minister needs to ensure that they are delivering key public services, rather than being distracted by the bureaucracy of major reorganisational change.”

Former international development secretary Andrew Mitchell warned against the abolition of his old department.

“DFID is the most effective and respected engine of development anywhere in the world, and a huge soft power asset for Britain,” said Mr Mitchell. “Any machinery of government changes in Whitehall should obviously respect Britain’s international development in the poorest and most unstable parts of the world. Tackling insecurity and building prosperity directly affects our well being in the UK. British leadership in this area is a core part of Global Britain.”

Meanwhile, Ms May’s former Europe adviser Raoul Ruparel said the government’s attention should be focused on the next phase of negotiations with the EU, which will determine Britain’s trade and security relationship with the continent for decades to come.

“The government and the civil service have a lot of work to do if they are to prepare to leave with a deal,” said Mr Ruparel. “As it stands, the UK does not appear ‘match fit’ for the next phase of negotiations.”

Rishi Sunak has refused to give details of the planned shake-up (Getty)

Chief secretary to the treasury Rishi Sunak declined to discuss the planned shake-up during an interview on BBC1’s Andrew Marr Show, saying only that the government wants to “innovate” to deliver for the public.

“I think that people watching will not be interested in the process of government, what they want to know is that government is going to deliver for them,” said Mr Sunak. “Are our schools improving, are we investing in transport infrastructure, is the NHS getting better?”

Michael Gove said he did not know whether reports that he would lead negotiations with the EU at the head of a new beefed-up trade department were correct, saying that this was a matter for the prime minister.

But he insisted that he was confident that a trade deal with the EU will be “concluded” by the end of 2020, despite Brussels chief negotiator Michel Barnier’s warning that it was unrealistic to expect a comprehensive agreement within such a tight timescale.

“We will have concluded our conversations with the EU about the new framework of free trade and friendly co-operation that we will have with them by the end of next year.”

Mr Gove – who is currently in charge of no-deal preparations as chancellor of the Duchy of Lancaster – told Sky’s Ridge on Sunday: “Quite a lot of the details that we need to negotiate are already laid out in the political declaration, so a lot of work has been done.

“And as a number of people have pointed out, there are areas where the European Union’s interests and the United Kingdom’s interests are already closely aligned, so I’m confident that we will be able not just to leave the EU on 31 January but also to conclude all the details of a new relationship in short order.”