Cabinet Secretary Sir Mark Sedwill has warned that civil service officials have become “fair game” in modern politics as he spoke out again on anonymous briefings against him.

Sir Mark, the most powerful official in Government, is set to step down as Cabinet Secretary and national security adviser (NSA) in September amid reports of clashes with the Prime Minister’s chief adviser Dominic Cummings.

He told MPs and peers on the National Security Strategy Committee that it was “never pleasant” to read off-the-record reports against civil servants in the media but said it had become a “regrettable feature of modern politics”.

We appear to be in an era where some of us are fair game in the media and I’m afraid it goes with the territory now

Before Sir Mark stood down, criticisms of his performance, particularly his handling of the coronavirus outbreak, appeared in the press.

“It is never pleasant to find oneself, particularly as an official, in the midst of stories of that kind,” he told the committee on Wednesday.

“We appear to be in an era where some of us are fair game in the media and I’m afraid it goes with the territory now.

“I don’t think it is ever pleasant in government, whether it is against ministers, between them and particularly against officials, when you have briefings to which you cannot really reply, particularly those that are off the record and sniping away.

“But it is a regrettable feature of modern politics, I’m afraid.”

His comments came on the same day that the civil service started advertising for a replacement cabinet secretary, with an advertised salary of £200,000.

It was also revealed that Sir Mark will be paid £248,000, which is likely to come “in the form of a pension contribution”, after the Prime Minister agreed to compensate him for “stepping down early”.

When asked why he resigned from his dual post, Sir Mark replied: “I haven’t resigned.

“The Prime Minister and I agreed I should step down – it was by agreement.

“That was essentially because we had concluded it was time to split the jobs again and have a separate security adviser and separate cabinet secretary.”

The former Home Office permanent secretary said “I don’t think personalities were an issue at all” during his time working in Downing Street and said he had a “really good relationship” with Boris Johnson.

Sir Mark said the timing of his departure was “at his initiative” but that it was “entirely amicable”.

“It was never my intention to do that (cabinet secretary job) long term,” the former ambassador added.

David Frost, currently Mr Johnson’s Europe adviser, will take over as NSA in the autumn.

Committee chair Dame Margaret Beckett MP put it to the top civil servant that his successor did not have the “degree of security experience” possessed by the outgoing incumbent.

But Sir Mark, who noted that past appointments had lacked his own domestic security background, heralded Mr Frost as a “very experienced diplomat” and pointed to the Brexit sherpa’s previous role in setting Foreign Office strategy.

“His job is not to be the expert but to draw on the expertise and represent the Prime Minister and the Government,” said the 55-year-old.

The mandarin also said it was “quite common” in other countries for the security adviser to be a “political appointee who is closely aligned with the head of government”.

Sir Mark, in what are likely to be some of his final public comments as NSA, called on China to be “transparent” during any investigation into the Covid-19 outbreak.

“You’re absolutely right, we do need transparency from China – the Prime Minister himself has spoken about this,” he told Commons defence committee chair Tobias Ellwood MP.

“This is not about finger-pointing but it is about understanding and understanding exactly what happened, how quickly the disease was transmitted, what is its genetic make-up, how did it move – if indeed it did – from bat to Pangolins to human, and are there things we can learn.”