THE ousting of the UK’s top civil servant could lead to a US-style politicisation of Whitehall, a former cabinet secretary has warned.

Lord O’Donnell said he was concerned about the departure of Sir Mark Sedwill after just two years and a political appointee taking over one of his former roles.

Lord O’Donnell said the danger was in the Prime Minister surrounding himself with “yes men” rather than neutral officials ready to speak truth to power.

His fear appeared to be borne out by Education Secretary Gavin Williamson, who defended one of the PM’s special advisers becoming the new National Security Advisor.

“That’s what you see in the United States, that’s what you see in a lot of countries,” he said.

Sir Mark announced on Sunday that he would stand down in September as cabinet secretary, national security adviser and head of the civil service, a decision that had been anticipated in the wake of weeks of negative briefings against him.

Opposition MPs pointed the finger of blame at the PM’s top aide, Dominic Cummings.

Mr Cummings reportedly wanted Sir Mark gone in order to pursue his long-cherished plan to overhaul the civil service, which he sees as a roadblock to reform.

Mr Johnson recommended Sir Mark for a peerage and a new role with the G7 and appointed his Europe adviser David Frost, a former chief executive of the Scotch Whisky Association, as the new national security adviser, despite his apparent lack of experience in the area.

Mr Frost, a former diplomat in Brussels, has also been recommended for a peerage.

He carries as the UK’s main Brexit negotiator.

In an interview with Times Radio today, Mr Johnson said Sir Mark still had a “lot to offer” and had given “incredible service” to the country at a difficult time.

He downplayed suggestions Sir Mark had been the subject of toxic briefings in the media.

"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," he said.

"People brief all kinds of things into the newspapers. All I can tell you is Mark is an outstanding servant to this country and will continue to be so."

But Lord O'Donnell said he was concerned over Sir Mark Sedwill's resignation from the role.

He told BBC Radio 4: "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, how that's consistent with [cabinet minister] Michael Gove's desire for deep subject knowledge for someone who hasn't really had much of a background in national security."

Asked if the service was being politicised and its impartiality eroded, the peer said: "I think appointing a special adviser to a national security role is quite clearly an example of that.

"It's a problem because political appointees are there and they are more likely to be subject to group-think, more likely to be yes-men, more likely to say what it is ministers want to hear as opposed to giving good, objective, speaking truth to power which is what it's all about."

Mr Williamson said it is not unusual for a special adviser to be national security adviser.

He told BBC Radio 4: “He will be, but that's not unusual. That's what you see in the United States, that's what you see in many other countries.

"This is a man who has impeccable public service, very much a background that similar people who have held this role in the past before have come from having worked in the Foreign and Commonwealth Office for an awful long time, so this isn't an unusual appointment."

He did not deny suggestions the next cabinet secretary would be a Brexiteer.

He said: "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.”

But Mr Johnson said "who knows what his or her views will be?".

On a construction site visit in west London he said: "The great thing about the civil service is that nobody should know, least of all me."

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."

Sir Mark's exit follows a number of changes at the top of the Civil Service in recent months.

The Foreign Office's most senior civil servant Sir Simon McDonald - seen as a Brexit critic - was told this month he had to step down before the department was merged with the Department for International Development (DfID).

Sir Philip Rutnam quit as the Home Office permanent secretary earlier this year after accusing Home Secretary Priti Patel of a "vicious and orchestrated briefing campaign".

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.