Theresa May has questioned why Boris Johnson picked a political appointee with “no proven expertise” as his new national security adviser (NSA).

The former prime minister pressed Cabinet Office minister Michael Gove to explain the decision to replace Sir Mark Sedwill with David Frost, currently Mr Johnson’s chief EU negotiator.

Mr Johnson has been criticised for the appointment as, unlike previous holders of the post, Mr Frost is a political adviser rather than a career civil servant – who also lacks security experience.

Sir Mark is also stepping down as Cabinet Secretary and head of the Civil Service, amid reports of clashes with Mr Johnson’s chief adviser Dominic Cummings.

Speaking during an urgent question in the Commons, Conservative MP Mrs May said: “I served on the National Security Council for nine years – six years as home secretary and three as prime minister.

“During that time, I listened to the expert independent advice from national security advisers.

Sir Mark Sedwill
Sir Mark Sedwill (House of Commons/PA)

“On Saturday (Mr Gove) said: ‘We must be able to promote those with proven expertise.’

“Why then is the new national security adviser a political appointee with no proven expertise in national security?”

Mr Gove responded: “We have had previous national security advisers, all of them excellent, not all of them necessarily people who were steeped in the security world, some of whom were distinguished diplomats in their own right.

“David Frost is a distinguished diplomat in his own right, and it is entirely appropriate that the prime minister of the day should choose an adviser appropriate to the needs of the hour.”

Mr Gove later insisted Mr Frost “is an expert” given his experience in diplomatic service.

For Labour, shadow home secretary Nick Thomas-Symonds questioned why this was a political appointment.

He said: “The first duty of any government is to keep people safe and, in carrying out that duty, any government should have objective and at times challenging advice from its national security adviser.

“And it’s why making a political appointment takes this Government into such dangerous territory.”

SNP home affairs spokeswoman Joanna Cherry said the decision to replace Sir Mark is all about the “revenge” of the Vote Leave campaign.

House of Commons
Cabinet Office minister Michael Gove answers a question from former PM Theresa May about the new national security adviser (House of Commons/PA)

Yvette Cooper, Labour chairwoman of the Home Affairs Committee, asked: “Will the new politically-appointed special envoy national security adviser be responsible for the performance reviews of the heads of MI5, MI6 and GCHQ?

“Does he agree that nothing should be done to suggest any political interference in the crucial intelligence agencies who support our national interest?”

Mr Gove said this was an “important point”, adding in the Commons: “Of course it’s the case, when we do have reviews of those in those roles, they are carried out by those who can be fully objective in the round and in a way which is free of any taint of political interference.”

Labour former minister Steve McCabe asked if Mr Frost will have finished with his duties as EU negotiator by the time he takes the security job, adding: “Or is it still this Government’s view that the national security adviser will be a part-time role?”

Mr Gove replied: “We’re confident that we’ll be making progress over the next few weeks in EU negotiations.

“They’re being conducted intensively, specifically at the request of the Prime Minister and the president of the European Commission.”

Conservative former Cabinet ministers Andrew Mitchell and Sir John Redwood backed the changes made by the Government, with Mr Mitchell telling the Commons: “It does seem to be, it’s clearly sensible to have the national security adviser separate from the head of the Civil Service.”

The SNP’s Margaret Ferrier (Rutherglen and Hamilton West) said there was “a toxic workplace environment for senior civil servants”, something Mr Gove denied.