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.

“It has been a privilege to serve.”

Sir Mark will step down from his role in September.

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. 

JUST IN: Coronavirus daily briefing: A look back at the most memorable momen…

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. 

“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.”