UK's top civil servant to quit after clash with Number 10

His exit, after weeks of briefing against him, helps Mr Cummings begin his long-cherished ambition to overhaul the civil service, which he sees as a barrier to reform.

Boris Johnson said he had recommended Sir Mark for a peerage alongside his Brexit negotiator David Frost, who becomes the new national security adviser.

Sir Mark has been asked to lead a new G7 panel on Global Economic Security as the UK assumes the presidency.

Labour said the Government should be focusing on Covid and the looming economic crisis not “reshuffling Whitehall”.

The SNP called it "reckless" and "blatant politicisation of the civil service".

Sir Mark announced his departure plans hours after the Sunday Telegraph reported he would leave imminently as part of a “Whitehall revolution” led by Mr Cummings.

Last week Mr Cummings told political aides “a hard rain is going to fall” after blaming Whitehall for a series of failures during the coronavirus crisis.

Sir Mark, 55, who studied at St Andrews, was appointed the national security adviser by Theresa May in 2017 and unusually kept the role after becoming cabinet secretary in 2018. 

However his star waned after Mr Johnson became PM, and speculation about his future has grown in recent weeks after the creation of a rival power centre in Downing Street.

Mr Johnson last month promoted Simon Case, a senior civil servant and private secretary  to Prince William, to the role of permanent secretary at Number 10.

Mr Case has been tipped to be the new cabinet secretary.

In his letter to Mr Johnson, Sir Mark said: "Two years ago, when my predecessor fell ill, your predecessor asked me to step in as Cabinet Secretary, and you asked me to continue to support you through Brexit and the election period.

"It was obviously right to stay on for the acute phase of the Covid-19 crisis.

"As you are setting out this week, the Government's focus is now shifting to domestic and global recovery and renewal."

In his responding letter, Mr Johnson praised Sir Mark for his “outstanding work” over 30 years inthe civil service. 

He said: “Over the last few years I have had direct experience of the outstanding service that you have given to the Government and to the country as a whole.

"It has been by any standards a massive contribution - but as PM I have particularly appreciated your calm and shrewd advice.

"You have also spoken with a unique authority - unusual in a Cabinet Secretary - on international affairs and national security; and as National Security Adviser you have done much to keep this country safe.

"It is therefore great news that you have agreed to continue to serve this country on the international stage, beginning with the UK's preparations for the G7 summit next year."

"You have done it all in Whitehall: from Afghanistan to the modernisation of the civil service; from immigration policy to Brexit and defeating coronavirus. 

“After serving for decades with great distinction - and unflappable good humour - I believe you have earned the gratitude of the nation."

Labour MP Helen Hayes, the shadow minister for the Cabinet Office, said: "We pay tribute to the work Mark Sedwill has done.

"He has been a dedicated public servant and has run the civil service in difficult times.

"On the day it was revealed millions of jobs across the country could be under threat in the coming months, it is very concerning that Boris Johnson and Dominic Cummings are preoccupied with reshuffling Whitehall."

SNP Westminster leader Ian Blackford said: “The independence of civil service – essential for good governance – is being completely undermined by Boris Johnson and his allies.  

“Dominic Cummings has been an outspoken critic of the civil service and appears now to have gotten his way.

“We have now seen a number of senior civil servants leave their roles – while a crucial national security job is filled not with a civil servant but a political appointee. 

“That is a blatant politicisation of the civil service – and raises yet more questions about Boris Johnson’s reckless approach to governance.”

The FDA, the body which represents civil servants, said the Government would be "weaker as a result" of Sir Mark's departure.

General secretary Dave Penman said: "Sir Mark Sedwill has been one of the outstanding public servants of his generation, serving the country's interests at home and abroad as a diplomat, weapons inspector, permanent secretary and ultimately Cabinet Secretary.

"Whatever emerges as fact from the series of briefings that have sought to undermine Sir Mark's position, this Government will emerge weaker as a result."

The FDA accused "No 10 - or those around it" of seeking to "undermine Sir Mark and the leadership of the civil service” with anonymous briefings against him over many months.

Mr Penman said: "Not only is it a self-defeating and corrosive tactic, it's also a cowardly one, safe in the knowledge that those who are briefed against are unable to publicly respond."

Sir Mark has clashed a number of times with Mr Johnson and his team.

Last November, he blocked publication of a costing of Labour’s election plans written by Treasury civil servants after then shadow chancellor John McDonnell complained.

The move infuriated the then Chancellor Sajid Javid.

Sir Mark reportedly held onto his post after Mr Johnson’s election win in December after assuring people he could help with civil service reform.

However Mr Cummings and cabinet minister Michael Gove appeared not to have been convinced.

Mr Cummings said earlier this year there were profound problems in how decisions were made and criticised a lack of expertise in specific fields in the service.

While Mr Gove said this week that if the government is to carry out the reforms it wants it also needs to reform itself.

he Sunday Telegraph quoted a friend of Sir Mark saying he had been “viciously briefed against”, adding: “The whole Gove-Cummings axis has been sowing discord between the Prime Minister and Mark Sedwill.”