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.