Theresa May has launched a forthright attack on Boris Johnson’s government over the appointment of the EU negotiator David Frost as the UK’s national security adviser. The former prime minister accused the Cabinet Office minister, Michael Gove, of promoting someone “with no proven expertise” to a crucial role at the heart of the UK’s safety. It follows an outcry from security officials and former senior civil servants at the decision to hand the job to Frost, who is expected to prioritise his role negotiating the UK’s deal with the EU while learning his new job. Though formerly a long-serving diplomat, Frost, known as “Frosty” among Johnson’s closest aides, has little direct experience of security matters. Unlike other national security advisers since the role was created in 2010, Frost is not a civil servant, but a political appointee. Frost will receive a peerage and replace Sir Mark Sedwill, who will also step down from his roles as cabinet secretary and head of the civil service, as part of a Whitehall shake-up announced on Sunday. May, who served on the national security council for nine years as home secretary and then prime minister, made her comments as Gove responded to an urgent question about Frost’s role in the House of Commons. She paid tribute to Sedwill, saying she had listened to expert independent advice from national security advisers for nine years, and noted that Gove had made a recent speech saying the government should be able to promote experts to key roles. “On Saturday, my right honourable friend 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?” she said. Gove replied: “We have had previous national security advisers, all of them excellent, not all of them necessarily were people who were steeped in the security world. Some of them were distinguished diplomats in their own right. David Frost is a distinguished diplomat in his own right.” As Gove spoke, May grimaced and shook her head. The former Labour minister Angela Eagle asked Gove to return to May’s question, saying he had failed to answer it. “What are his specific qualifications and expertise and why on earth was he considered for a second for this role?” she said. Gove replied: “The broader point is that David Frost is involved in one of the most complex diplomatic negotiations ever conducted and a diplomatic negotiation that relates specifically to defence and security cooperation as well as tariffs and trade. “He has been a civil servant for decades and it is the case that Mark Lyall Grant and Kim Darroch, who were national security advisers, were not people who were steeped in the world of intelligence and security. .” It is the first time that May has launched an attack on Johnson’s administration since she was forced from office last summer. She appointed Sedwill to the role as national security adviser in 2017. Later, the defence secretary, Ben Wallace, backed Gove, saying in front of a defence select committee that it was not necessary for Frost to have a national security background, arguing instead that his experience dealing with Brussels would help. “I think negotiating with the EU sets you up pretty well to tackling an octopus which the national security construct can sometimes be,” the minister added. Wallace also said that Sedwill would “make a fantastic secretary general of Nato if he is successful in getting that appointment”, making him the first minister to publicly confirm that the outgoing cabinet secretary and national security adviser would be put up by the UK as a candidate for the top job at the military alliance next year. Frost, 55, will be the secretary of the 15-strong National Security Council, which meets regularly to discuss strategy. Meetings are chaired by the prime minister and include the heads of MI5 and MI6, GCHQ, and the country’s most senior military officers. The SNP’s home affairs spokeswoman, Joanna Cherry, said Sedwill has been forced out because he warned last year that Brexit could be a disaster. “So this is all about the revenge of the Vote Leave campaign, whose so-called mastermind [Dominic Cummings] is now pulling the strings of this government. “Although one does have to wonder about the masterly-ness of a mind that thinks a good way to test one’s eyesight is to go for a 60-mile drive,” she said. Despite occupying what was once a neutral role, Frost could be introduced to parliament as a Conservative, a Tory minister confirmed. Lord True told the House of Lords: “It is my understanding that he may be introduced as a Conservative peer, but I can’t confirm that.” In Gove’s speech on Whitehall reform on Saturday, he called for a push to retain expert advisers. “There are systemic problems which mean we often lose institutional memory and fail to build on hard-won success,” he said. Sedwill, the UK’s most senior civil servant, will step down from Downing Street in September and has also been handed a peerage. Johnson has discussed the possibility that Sedwill could be officially nominated to become Nato’s next secretary general. However, the post is not likely to become free until the end of 2022, when the incumbent, Jens Stoltenberg, is expected to retire after eight years in the role.