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.
Theresa May through the years
Theresa May through the years
Britain's Prime Minister Theresa May leaves 10 Downing Street, as she faces a vote on alternative Brexit options, in London, Britain, March 27, 2019. REUTERS/Alkis Konstantinidis
British Prime Minister Theresa May answers a question after making a statement in the Parliament in London, Britain March 25, 2019, in this still image taken from video. Reuters TV/via REUTERS
Britain's Prime Minister Theresa May leaves church, near High Wycombe, Britain March 24, 2019. REUTERS/Henry Nicholls
Britain's Prime Minister Theresa May leaves after giving a news briefing in Brussels, Belgium March 22, 2019. REUTERS/Toby Melville
European Commission President Jean-Claude Juncker embraces with Britain's Prime Minister Theresa May attend a European Union leaders summit in Brussels, Belgium March 21, 2019. REUTERS/Yves Herman TPX IMAGES OF THE DAY
Britain's Prime Minister Theresa May makes a statement about Brexit in Downing Street in London, Britain March 20, 2019. Jonathan Brady/Pool via REUTERS
French President Emmanuel Macron welcomes Britain's Prime Minister Theresa May for a lunch at the Elysee Palace as part of the One Planet Summit in Paris, France, December 12, 2017. REUTERS/Philippe Wojazer
FILE PHOTO - Foreign Secretary Boris Johnson attends a cabinet meeting hosted by Theresa May at the Prime Minister's country retreat Chequers in Buckinghamshire to discuss department-by-department Brexit action plans, Britain August 31, 2016. REUTERS/Stefan Rousseau/Pool/File Photo
British Prime Minister Theresa May during the Conservative Spring Forum 2017 held at the SSE SWALEC Stadium in Cardiff, Wales. Picture date: Friday March 17th, 2016. Photo credit should read: Matt Crossick/ EMPICS Entertainment.
Home Secretary Theresa May arrives for the Conservative Party conference at Manchester Central.
Home Secretary Theresa May delivers her speech to the Conservative Party conference at Manchester Central.
Home Secretary Theresa May stands during a silence honouring three fallen police officers at the annual Police Federation Conference in Bournemouth.
Home Secretary Theresa May during her speech to delegates at the Conservative Party annual conference in the International Convention Centre, Birmingham.
Home secretary Theresa May as she addresses the Conservative Party conference in Manchester.
Home Secretary Theresa May at the Conservative Party Conference at International Convention Centre in Birmingham.
Britain's Home Secretary Theresa May leaves after attending the funeral service of former British prime minister Margaret Thatcher at St Paul's Cathedral, in London April 17, 2013. Thatcher, who was Conservative prime minister between 1979 and 1990, died on April 8 at the age of 87. REUTERS/Olivia Harris (BRITAIN - Tags: POLITICS RELIGION OBITUARY SOCIETY)
Home Secretary Theresa May addresses the Conservative Party conference at the International Convention Centre in Birmingham.
Home Secretary Theresa May arrives at Downing Street for a cabinet meeting ahead of today's Budget statement.
British Home Secretary Theresa May meets Mounted Police Officers PC Kate Leake (right) and Sergeant Darren Duffy (left) in the Olympic Park, on day 15 of the London 2012 Olympics.
Britain's Home Secretary Theresa May listens as Britain's Prime Minister David Cameron (not pictured) delivers his speech at the Lord Mayor's Banquet in the City of London November 14, 2011. Traditionally, the prime minister makes a major World Affairs speech at this occasion. REUTERS/Toby Melville (BRITAIN - Tags: POLITICS)
Britain's Home Secretary Theresa May waits as she is introduced before delivering her speech on police reform, in central London August 16, 2011. May said reforms to policing would allow more officers to patrol the streets and added that she could widen their curfew powers to prevent a repeat of the recent riots. REUTERS/Andrew Winning (BRITAIN - Tags: POLITICS CRIME LAW)
Home Secretary Theresa May attends the Women of the Year Lunch and Awards 2010, at the Intercontinental Hotel, in London.
Britain's Prime Minister David Cameron and Home Secretary Theresa May look at passports during a visit to UK Border Agency staff, in Terminal 5 at Heathrow Airport, London November 23, 2010. Britain said on Tuesday it would cap the number of skilled migrant workers it allows from outside the European Union at 21,700 a year, a cut of more than a fifth from 2009 levels. REUTERS/Steve Parsons/Pool (BRITAIN - Tags: POLITICS SOCIETY)
Britain's Prime Minister David Cameron and Home Secretary Theresa May (L) walk through Terminal 5 during a visit to UK Border Agency staff, at Heathrow Airport, London November 23, 2010. Britain said on Tuesday it would cap the number of skilled migrant workers it allows from outside the European Union at 21,700 a year, a cut of more than a fifth from 2009 levels. REUTERS/Steve Parsons/Pool (BRITAIN - Tags: POLITICS SOCIETY)
Britain's Conservative Party Shadow Leader of the House of Commons, Theresa May, wears boots as she speaks during the first day of the Conservative Party annual conference in Blackpool, northern England September 30, 2007. REUTERS/Toby Melville (BRITAIN)
Theresa May MP arrives at the Conservative Party Black & White Ball at Old Billingsgate Market, Wednesday 8 February 2006. PRESS ASSOCIATION Photo. Photo credit should read: Michael Stephens/PA
Shadow Minister for Culture, Media and Sport and Family, Theresa May, conducts a radio interview in the lobby of the Imperial Hotel, Blackpool, Tuesday October 4, 2005, on the second day of the Conservative Party annual conference. The Tories should rebrand themselves as the Modern Conservative Party, one of its most successful campaigners has suggested. Lord (Tim) Bell, who ran hugely successful election campaigns for Margaret Thatcher two decades ago, argued that a distinctive new look is needed in order to demonstrate to voters that the party has changed since it lost power. See PA TORY Stories. PRESS ASSOCIATION Photo. Photo credit should read: Phil Noble/PA
Theresa May MP.
Sandra Howard, (3rd left) wife of Conservative Party leader Michael Howard, is joined by Conservative Party Shadow Minister for the Environment and Transport Theresa May, (3rd right), Barrister Emma Broadbent (2nd left), Lady Helen Young (left), Chair of the Equal Opportunities Commission Julie Mellor (2nd right) and Aminah Bhatti, from Bradford, at the Asian Women of Achievements Awards held at the Park Lane Hilton in central London. The evening, the fifth of its kind, recognises and celebrates the extraodinary contribution of asian women who have enriched the social fabric of the society they live in.
Former Conservative party leader Iain Duncan Smith (2R) poses for
photographs with (L to R) shadow Secretary of State for Defence Bernard
Jenkin, shadow Secretary of State for the Office of Deputy Prime
Minister David Davis, Conservative party chairman Theresa May and
shadow Foreign Secretary Michael Ancram at the launch of his debut
novel "The Devil's Tune" in London, November 6, 2003. Britain's
Conservative party ushered in veteran former minister Michael Howard as
its new leader on Thursday in an attempt to end years of in-fighting
and mount a credible challenge to Prime Minister Tony Blair.
Conservative party leader Iain Duncan Smith holds up the hand of
chairman Theresa May after her speech at the party conference in
Blackpool, October 6, 2003. REUTERS/Ian Hodgson
Shadow Education Secretary Theresa May during her speech at the Conservative Party Conference in Bournemouth. *23/07/02 Theresa May who has been named Tuesday July 23, 2002, as the new Conservative Party chairman. She replaces David Davis in a reshuffle by Iain Duncan Smith of his shadow cabinet.
Shadow Education Secretary Theresa May outside Conservative Central Office in London, in the run up to the June 7 general election. May attacked the government's education policies and said that the Tories would cut through red-tape to make it easier for teachers.
Barbara (Paddu) Beels, (right) from Wingate Nursery School, Wingate, County Durham, with the Camelot Award for Working with Parents and the Community in a Primay School. North East Region, presented to her by MP Theresa May, at The 1999 Teaching Awards. * sponsored by Lloyds TSB at Alexandra Palace in London.
Theresa May, Conservative Candidate from Lanchester, North West Durham. England. Pictured taken 14th March 1992. (Photo by NCJ Archive/Mirrorpix/Getty Images)
Theresa May, Conservative Candidate from Lanchester, North West Durham. England, Pictured 14th March 1992. In this 1992 picture, Theresa May was Conservative candidate fighting to be North West Durham MP in the 1992 General Election which was held on 9th April 1992. Result of the 1992 North West Durham was Labour - Hilary Armstrong received 22,734 votes. Cons - Theresa May received 12, 747 votes Lib Dem - Tim Farron received 6,728. (Photo by NCJ Archive/Mirrorpix/Getty Images)
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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.
"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."