He has been called the most powerful man in the UK, a securocrat who is too smooth for his own good.

But few would dispute that the influence of the cabinet secretary, Sir Mark Sedwill, has increased since Boris Johnson was admitted to St Thomas’ hospital with coronavirus on Sunday night.

Proof of his crucial role in Downing Street came in the drama of Monday night – when it fell to Sedwill to inform a stunned cabinet that the prime minister had been moved to an intensive care unit.

Sedwill will now be the primary adviser to Dominic Raab, the foreign secretary who is deputising for Johnson, as he confronts the world’s biggest health crisis in a century.

His pivotal role behind the scenes – Sedwill is also the national security adviser and head of the civil service – has made him a target for sniping in recent weeks.

Unnamed sources have claimed in national newspapers that he is failing to get a grip on the coronavirus crisis; some quoted in the Financial Times even claimed that he had fallen out with Johnson and his aides over the response to the virus.

On Tuesday, in unusually forthright language, the criticism was categorised as “shit-stirring” and “absolute crap” by Cabinet Office insiders.

One source said: “Suggestions that there are tensions between the PM and Mark are absolute crap. Naturally it is disappointing that some people choose to indulge in shit-stirring at a time of national crisis.”

Sedwill, 55, is aware that there have been political briefings against him, but does not comment publicly.

An ally of the cabinet secretary said: “Of course he is aware of these stories but he is totally focused on working with his senior team and giving the cabinet, including Dominic Raab and the PM, his best advice at this incredibly challenging time.”

Pressure will build upon Sedwill, who attends cabinet and Cobra meetings, to coordinate government without the prime minister.

With Raab seen by some colleagues as a “stiff” media presence and without the common touch or experience of Johnson, a one-time Whitehall critic of Sedwill said the cabinet secretary’s role in guiding Raab will be crucial.

“Right now, the country needs Sedwill until Boris comes back,” the source said.

Allies of Sedwill say he thrives in high-pressure environments – he worked for many years in Afghanistan – and has managed to continue working from his office overlooking Horse Guards Parade through the current crisis. Many from Downing Street, including the prime minister’s adviser Dominic Cummings, are self-isolating.

With a private office of about half a dozen staff, Sedwill arrives at 6.30am and rarely leaves before the evening. During the current crisis, Sedwill has attended Covid committee meetings at 9.15am, Cobra and Sage meetings and has spoken to the prime minister several times a day.

The cabinet secretary coordinates the work of permanent secretaries as they grapple with lockdown, PPE supplies, food supplies, prison releases and coronavirus tests.

“He does not take notes, but he steers the discussion,” said one former attendee.

Occasionally abrupt, he has also made enemies in government. The education secretary, Gavin Williamson, was sacked from his previous job as defence secretary after Sedwill led a probe into a leak from the National Security Council and concluded that Williamson was the source.

Some senior civil servants have claimed that he has failed to back the Home Office’s former permanent secretary Sir Philip Rutnam in the ongoing row over allegations that the home secretary, Priti Patel, has bullied staff.

Sedwill was born and grew up in Lincolnshire, attending Bourne grammar school. He studied international economics at the University of St Andrews and has a master’s in economics from St Edmund Hall, Oxford.

After graduation in 1987, he worked as a scuba diving instructor while travelling, which almost prompted him to abandon hopes of a more formal career. After joining the Foreign Office in 1989, he had postings in Afghanistan, Egypt, Iraq and Pakistan.

With a CV suggesting connections with MI5 and MI6, Sedwill has been portrayed by enemies as a “securocrat” – a stiff-collared spook happier in the company of spies and soldiers rather than gossiping with politicians and journalists.

Allies say he is happiest relaxing with friends and family and admit that his time working closely in national security may have resulted in a permanent secretary who is ultra-cautious about giving away too much and rarely lunches in Westminster.

Robert Hazell, professor of government and the constitution at the constitution unit, University College London, said: “He will be a strong hand steadying the ship and adviser to Dominic Raab.”