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.
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.”
More to follow…