The UK’s top civil servant, Simon Case, has told the UK Covid-19 Inquiry that he found Boris Johnson’s style of working “very frustrating” and described his day-to-day administration as “dysfunctional”.

Giving evidence on Thursday morning, the Cabinet Secretary discussed his deep “regret” over sending scathing WhatsApp messages about Mr Johnson during the pandemic.

He added he did not initially understand how “personally” difficult Mr Johnson found it to introduce lockdowns and described him as a “great communicator”.

Questioned by lead counsel to the inquiry Hugo Keith KC about Mr Johnson’s prime ministerial style, Mr Case replied: “Each prime minister has their own approach to doing it and as I say, in my job, I found it very frustrating.

“I just don’t think I understood how difficult he was finding it personally.”

Mr Case added: “I don’t think I really understood how at quite a deep ideological level the prime minister found the mass locking up of the population – the harms.

“He was always particularly focusing on children and education, and the damage as he saw it that was being done to society through big decisions on the lockdowns.”

Asked about the day-to-day administration of government, Mr Case said: “It was definitely dysfunctional and it was difficult, oddly enough, sclerotic isn’t quite the right word.

“If anything, the problem is it was almost too dynamic.

“It was difficult to settle on the course of action, and be sure that the course of action would be consistent. I think that’s almost the opposite of sclerosis.”

In July 2020, before he became Cabinet Secretary, Mr Case said “I’ve never seen a bunch of people less well-equipped to run a country” in a message to Lord Sedwill, who was the Civil Service chief at the time.

He also described Mr Johnson and his inner circle as “basically feral” and suggested the then-prime minister’s wife Carrie was “the real person in charge” in No 10.

Responding to negative WhatsApp messages he sent about Mr Johnson, the Cabinet Secretary told the inquiry: “That material is there.

“Those are examples that I obviously now deeply regret of expressing my in-the-moment frustrations with the former prime minister.”

Mr Case’s appearance at the inquiry came a day after Rishi Sunak called a general election, following the Cabinet Secretary’s return to work after he stepped back in October due to a medical matter.