Boris Johnson has been criticised for handing one of Britain's top security roles to a political ally rather than a civil servant.
It comes after Britain's top mandarin Sir Mark Sedwill announced he was retiring from the role alongside the post of Cabinet Secretary, after months of brutal briefing against him from inside Downing Street.
The shake upwill see Sir Mark's role as national security adviser filled by the PM's Europe adviser David Frost - a political appointment rather than an impartial civil servant.
The post, created by David Cameron, is supposed to sit at the top of the UK governments intelligence and security structure, ensuring that the Prime Minister gets the clearest possible picture of the challenges facing the country from Britain's network of diplomats, spies and signals intelligence.
All four previous holders of the post have been senior career civil servants, with extensive backgrounds in security.
Ex-cabinet secretary Lord Gus O'Donnell said: "I'm worried about the appointment of David Frost as national security adviser because I'm not quite sure how putting a special adviser in that role works."
He told BBC Radio 4's Today that political appointees were "more likely to be yes-men" rather than "speaking truth to power".
As if to confirm Lord O'Donnell's fears Education Secretary Gavin Williamson has suggested the appointment of Mr Frost as national security adviser would move the country towards a US-style model of political appointments.
He told BBC Radio 4's Today programme it was "not unusual" as "that's what you see in the United States, that's what you see in many other countries".
He also did not deny suggestions that Mr Johnson would fill the Cabinet Secretary vacancy with a Brexiteer.
"The Prime Minister's search is to get the very best person into that role and applications will be opening from next month to get someone of the very highest calibre to step into some big shoes," he told Today.
The Prime Minister's Official Spokesman said: "The appointment of the NSA is always the role of the Prime Minister. It's not unusual in other countries to have ambassadors serve as National Security Advisers and ambassadors can be political appointees.
"David Frost has the status of an ambassador."
He added: "The First Civil Service Commissioner has agreed the appointment and that is consistent with the constitutional reform and governance act."
Opposition MPs have pointed the finger at the Prime Minister's chief aide Dominic Cummings, suggesting he played a role in Sir Mark being forced out as head of the Civil Service.
Mr Cummings is rumoured to have a difficult relationship with Sir Mark, who was appointed Cabinet Secretary in 2018 by Theresa May, with the former PM allowing him to unite the role with his national security adviser job.
Dave Penman, general secretary of the FDA union, which represents senior officials, said: "No 10, or those around it, has sought to undermine Sir Mark and the leadership of the Civil Service, with a series of anonymous briefings against him over many months."
He blasted the tactics as "corrosive and cowardly" and said the Government would be "weaker as a result" of the departure.
Asked about the departure of such a senior figure, Mr Johnson suggested Sir Mark Sedwill still had "a lot to offer" as he faced a storm over the resignation of the country's top senior civil servant.
The Prime Minister played down the "negative briefing" and said he did not attach the "utmost credence" to the reports.
Mr Johnson told Times Radio: "He has seen the Government through all sorts of very tough stuff - changes in the premiership, an election, Brexit, dealing with the worst bits of the Covid crisis. He has got a lot more to offer and I am sure he will."
In response to the reports surrounding Sir Mark's exit, he added: "I try not to read too much of the negative briefing.
"There is an awful lot of stuff that comes out in the papers to which I wouldn't automatically attach the utmost credence."
Labour leader Sir Keir Starmer told Sky News: "It seems to me obvious that the Prime Minister wanted to move the Cabinet Secretary and was determined to do so.
"Why you do so in the middle of a pandemic and a crisis instead of actually focusing on the crisis, is a question the Prime Minister needs to answer."
Acting leader of the Liberal Democrats, Sir Ed Davey, said: " Boris Johnson is clearly ready to grant Cummings his every wish when it comes to politicising the Civil Service and sweeping out those who may try to hold his Government to account."
The process for recruiting a new head of the Civil Service will start shortly, with applications to be invited from existing and former permanent secretaries.
