Theresa May has criticized the Prime Minister's appointment of his Brexit negotiator as the new national security advisor.
When speaking in the House of Commons, she suggested that David Frost lacked the expertise or independence to follow Sir Mark Sedwill in this role.
Labor said the election of Mr Frost, who is currently leading Britain's trade talks with the EU, is "dangerous".
But Cabinet Minister Michael Gove said Mr. Frost was highly qualified and accountable to the Prime Minister.
Mr. Frost is a former official who left the Federal Foreign Office in 2013 to work in the private sector.
He is expected to take on the role in late August. From then on, Sir Mark will also resign from his other role as Cabinet Secretary, the UK's top official.
Ms. May, who appointed Sir Mark Sedwill Prime Minister in both roles, told MPs that during her nine years as a member of the National Security Council, she "listened to the competent, independent advice of national security advisers."
She asked Mr. Gove how the appointment of Mr. Frost matched the comments he made in a recent public service reform speech on the need to "support people with proven expertise".
"Why is the new national security advisor a political official with no proven expertise in national security?"
"Needs of the hour"
Mr Gove said that there were precedents for non-officials who could play key roles in the Prime Minister's service and that the official who agreed to appoint officials abroad was appropriate in this case.
He said that Mr. Frost was neither an official nor a special adviser, but had the status of an ambassador.
"We have had excellent national security advisors in the past, not all of whom were rooted in the national security world, but rather well-respected diplomats," he said.
"David Frost is a respected diplomat himself, and it is perfectly fitting that the prime minister of the day selects an adviser who meets the needs of the hour."
But Ms. May shook her head violently as Mr. Gove spoke.
The two have quarreled in the past, primarily because of immigration when they were members of David Cameron's cabinet. Ms. May fired Mr. Gove when she became prime minister in 2016, but brought him back later.