Sir Mark Sedwill is stepping down as both Cabinet Secretary and National Security Adviser in September, with his dual roles separated into two separate positions. In his resignation letter to the prime minister, published on Sunday afternoon, Sedwill wrote:

“As we have discussed over the past few months, while the combined model was right for the circumstances of my tenure, as you lead the country through this next phase, you will need a separate National Security adviser to support you on this global agenda, permitting the Cabinet Secretary to focus on the domestic agenda.They should see you through the rest of this Parliament. Therefore, we have agreed that I will stand down and leave government service at the end of September. . . 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.”

In a generous, hand-written response, the Prime Minister thanked Sedwill for “the outstanding service that you have given to the government and to the country as a whole.” He added: “As PM I have particularly appreciated your calm and shrewd advice, as well as the many useful and amazing notes you have scribbled to me in Cabinet. 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.”

Although Sedwill will leave the heart of government, he will continue to represent the United Kingdom on the international stage. His future duties include chairing the G7 panel on global economic security as the UK assumes the presidency, and preparing for next year’s G7 summit, which the UK will host.

In his place as National Security Adviser has been appointed David Frost, who is currently serving as the government’s chief negotiator in Brexit trade talks. He is described as a “political appointment” in No10’s press release because he currently serves – and will continue to serve – in the capacity of special adviser to the prime minister, rather than civil servant, making him the first ever National Security Adviser whose hands aren’t tied by the civil service code. Having previously served as special adviser to Johnson in the Foreign Office, Frost’s loyalty lies squarely with the Prime Minister and, by extension, his political goals.

That is not to say that he is a purely political creature. On the contrary, the vast majority of Frost’s time in government has been as a career diplomat. He first joined the foreign office in 1987 and served in embassies and high commissions in Nicosia, Brussels, and New York. He then returned to London to serve as  private secretary to the head of the diplomatic service, John Kerr, now Lord Kerr, before becoming the British ambassador to Denmark in 2006. After again returning to London, in 2008, he quickly became a senior Whitehall official – first as director for strategy and policy planning in the foreign office, and then as director for Europe, Trade and International Affairs in the department for business, which made him the most senior trade policy official at the time.

Frost is a unique diplomat in that he politically understands Boris Johnson and his chief adviser Dominic Cummings. As the country endured the tumultuous run up to the 2019 Brexit negotiating deadline, a bond of trust was built between the threesome. They worked well under pressure, with Johnson and Cummings facing down constitutional barriers at home while Frost negotiated away red lines in Brussels, and ultimately achieved what many thought was impossible: the removal of the Northern Ireland backstop from the withdrawal agreement. In this tortuous process, the press got not a single hint of disagreements between the career diplomat and his political overlords. Frost wasn’t merely a servant of Downing Street but an active and faithful player within it.

Frost will now oversee national security, international strategy, and continue to lead the British delegation in Brussels trade talks. Through this loyal, powerful foreign policy tsar, the prime minister and his chief adviser will seek to impose their vision of Global Britain.