David Frost has "no proven expertise in national security", Theresa May has said in a blistering attack on Number 10’s Whitehall shake-up.
The former Prime Minister made her most adversarial appearance in the Commons since she became a backbencher, criticising the government’s decision to remove its most senior civil servant.
Mr Frost, currently the UK’s chief Brexit negotiator, will replace Sir Mark Sedwill as National Security Adviser - the first political appointee to the role since its creation in 2010.
Speaking in the Commons, Mrs May, who appointed Sir Mark to the role, described the “expert independent advice” she received from national security advisers in the nine years she spent on the national security council, first as Home Secretary then as Prime Minister.
She said "On Saturday [Michael 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?"
Mrs May’s attack on the government will be seen as a personal defence of one of her closest allies while she was in government.
Sir Mark, a former ambassador to Afghanistan, was appointed Political Director to the Foreign and Commonwealth Office in 2010, while William Hague was Foreign Secretary.
In 2013 he was appointed Permanent Secretary to the Home Office, while Mrs May was Home Secretary. As Prime Minister she made Sir Mark National Security Advisor in 2017, before giving him the additional role of Cabinet Secretary a year later.
Responding to Mrs May, Mr Gove, minister for the Cabinet office, said: "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."
The head of the civil service’s departure on Sunday came amid reports of increasing clashes with Dominic Cummings, the Prime Minister’s chief aide.
Mr Gove said the decision to separate out the NSA role from Cabinet Secretary was agreed some time ago, and praised Sir Mark’s “exemplary” service.
Mrs May made her intervention following an urgent question by shadow home secretary Nick Thomas-Symonds over the Whitehall shakeup.
Mr Thomas-Symonds questioned by the new NSA was a political appointee, and raised concern at reports earlier this year that Downing Street had a “hit list” of permanent secretaries they wanted to replace.
The Prime Minister’s spokesman said: “It is not unusual in other countries for ambassadors to serve as national security advisers and ambassadors can be political appointees. David Frost has the status of an ambassador.
“The First Civil Service Commissioner has agreed the appointment.” Mr Gove said that when David Cameron decided to create the role while he was in opposition, he had a political appointee in mind. However under the coalition government, Lord Ricketts was chosen instead, Mr Gove added.
Meanwhile Lord Ricketts said of Mr Frost's appointment: “Those advising ministers on national security do need the mastery of deep knowledge at a time when the government is formulating a new national strategy in a dangerous world.
“But the message of Frost’s appointment is that the prime minister accords absolute priority not to expertise and experience, but to political loyalty among his closest advisers,” Lord Ricketts wrote for the Royal United Services Institute for Defence and Security Studies, the think tank.
Ben Wallace, the Defence Secretary, praised Sir Mark as a “hugely talented person” adding he would make a “fantastic” secretary-general of Nato.
The government is expected to back Sir Mark’s candidature to succeed Jens Stoltenberg as part of an exit package agreed with the outgoing cabinet secretary.