Theresa May challenged the decision to appoint Brexit negotiator David Frost as national security advisor despite him lacking the “expertise” required. The Government announced the appointment on Sunday after confirming the incumbent Sir Mark Sedwill will step down in August. The former Prime Minister challenged Michael Gove over the decision as Mr Gove delivered an update on Sir Mark’s departure.

Mrs May, who counted Sir Mark as one of her closest advisers during her tenure, began by paying tribute to his work before launching into a tirade against Mr Gove and the Government.

She said: “May I first of all pay tribute to Sir Mark Sedwill and thank him for his extraordinary public service over many years.

“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 advisors.”

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Mrs May continued: “On Saturday, my Right Honourable Friend said, ‘we must be able to promote those with proven expertise.’

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

But Michael Gove rebuked Mrs May’s attack pointing out David Frost would not be the first national security advisor drawing his expertise from diplomacy rather than security world.

The Chancellor of the Duchy of Lancaster said: “Like my Right Honourable Friend, I too want to pay tribute again to Sir Mark and I appreciate, having served in Cabinet when she was Prime Minister, just how much we all owe to him for his distinguished public service.

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“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.

“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.”