Sir Mark, 55, announced his decision to step down this evening amid Boris Johnson’s plans to reshape the civil service. The UK’s chief Brexit negotiator, David Frost will succeed Sir Mark as National Security Adviser. In his letter to the Prime Minister, he 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.
“I am fortunate to have served in some of the most challenging and rewarding jobs in national and international public service under seven prime ministers and in extraordinary times.
“I am grateful for your confidence and friendship as both Foreign Secretary and Prime Minister. I wish you well and, of course, remain at your disposal in the years ahead.
UK news: Mark Sedwill resigns
UK news: Sir Mark will step down in September
“It has been a privilege to serve.”
Sir Mark will step down from his role in September.
In response, the Prime Minister 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.
UK news: He will step down from the two positions
“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.”
This comes amid plans to shake-up of the civil service lead by his chief adviser, Dominic Cummings.
Sir Mark also becomes the third high-level civil servant to have left their post since the turn of the year.
Sturgeon warning: Scottish voters told to ditch ‘isolationist’ SNP … [Latest]
Brussels infuriates Macron with plot to back down on fisheries [Update]
Brexit bombshell: Tony Blair predicts no deal Brexit [Latest]
UK news: Mr Frost has spearheaded negotiations
Philip Rutnam is suing the Home Office for unfair dismissal after leaving in February.
Simon McDonald is leaving the Foreign Office in September following the merger with the Department of International Development.
Home Secretary, Priti Patel hinted Whitehall changes would soon begin today.
She told Sky News: “This is the people’s government, delivering on the people’s priorities, and effectively any reforming government will be based around the type of delivery that our prime minister wants to drive for our great country and obviously have the right kind of support around him to deliver that.”
Some have stated the move is a “golden opportunity” to recreate both roles.
Peter Ricketts, who served as the UK’s first National Security Adviser from 2010 to 2012 welcomed the split.
UK news: Sir Mark sent the letter today
“It is a golden opportunity to recreate the badly-needed role of national security adviser.
“It was more than a full-time job when I did it and the world is an order of magnitude more dangerous and unpredictable now.”
Additional reporting by Macer Hall.