Boris 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.
Sir Mark is standing down from his role as both Cabinet Secretary and the UK’s national security adviser amid reports he was being undermined by Mr Johnson’s allies.
The Prime Minister played down the “negative briefing” and said he did not attach the “utmost credence” to the reports.
Former mandarins have criticised the shake-up, which will 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.
Ex-cabinet secretary Lord 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”.
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.
Mr Johnson showered praise on Sir Mark, telling 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.”
Education Secretary Gavin Williamson suggested the appointment of David Frost as national security adviser would move the country towards a US-style model of political appointments.
Defending David Frost’s position, 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.
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.”
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.
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.”
Mr Johnson said “Dom is outstanding” and “I have many excellent advisers and I’m delighted by the work that they do”.
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.
Simon Case, appointed No 10 permanent secretary in May and favoured over Sir Mark to lead the review into the two-metre social distancing restrictions, has been heavily tipped to take the top position.
Sir Mark’s exit follows on from 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).
Meanwhile, Sir Philip Rutnam, who was the Home Office’s permanent secretary, quit earlier this year after accusing Home Secretary Priti Patel of a “vicious and orchestrated briefing campaign” against him.
As part of agreeing to depart in September, Mr Johnson has nominated his outgoing Cabinet Secretary for a life peerage, and has also asked Sir Mark to lead a new G7 panel on global economic security as the UK prepares to assume the presidency next year.
Mr Frost has also been nominated by the PM for a life peerage.