The Prime Minister tested positive for the disease on March 27, and the top civil servant found out that he also had the virus on April 2, but this was only revealed six weeks after the fact.
The revelation also means that Mr Johnson, Sir Mark, Health Secretary Matt Hancock and the PM’s top aide Dominic Cummings were all battling coronavirus at a similar time, putting the normal functioning of Government at risk.
The late disclosure by Number 10 has raised questions among journalists over why the information was not made public earlier.
Sky News’ Deputy Political Editor Sam Coates said at the time of Mr Johnson testing positive, that he had been asking Downing Street ‘on a daily basis’ if Sir Mark Sedwill had coronavirus.
‘And every day we were told “he is fine, he is working as normal”,’ he said, ‘so to discover today from Downing St that he actually had it is something of a surprise, it might raise questions about the reliability of some information that comes out of Downing Street.
‘It is always disappointing when you end up discovering that something you had been asserting earlier were just wrong, but you know, it was for whatever reason a decision taken by somebody in Whitehall that the public weren’t to know that Sir Mark was ill.
‘We learnt today that he only ever had mild symptoms, and I don’t know whether or not people will take that as a justification given the way that this virus operates…’
Though not particularly well known, Sir Mark’s role as Cabinet Secretary makes him the second most powerful figure in government, executing the Prime Minister’s will on a daily basis.
Number 10 was grilled almost daily about the health status of key government players before and after Mr Johnson tested positive amid concerns about who was running the country.
But today, the Prime Minister’s Official Spokesman said he had only become aware ‘recently’ of Sir Mark having had coronavirus after he was told by a journalist.
Sir Mark Sedwill, pictured with Boris Johnson inside Number 10 on July 24 last year, had coronavirus at the same time as the PM
Number 10 today said Sir Mark, pictured alongside Mr Johnson and Rishi Sunak on April 28, had ‘continued to carry out his duties in full’ while working from home during his illness
Those now known to have had coronavirus at the heart of government at approximately the same time include Mr Johnson, the Health Secretary Matt Hancock and the PM’s top aide Dominic Cummings.
Chief Medical Officer Professor Chris Whitty also self-isolated after developing symptoms.
Told that Number 10 had previously always insisted Sir Mark was fine, the spokesman said: ‘Your recollection is entirely right.
‘You were asking me about the Cabinet Secretary and I said throughout that he was working as normal which he was.
‘He continued to carry out his duties in full so I was only aware very recently when it was pointed out to me by a journalist.
The revelation also means that (from left) Health Secretary Matt Hancock, Sir Mark Sedwill, and Boris Johnson were all battling coronavirus at a similar time, putting the normal functioning of Government at risk
‘The Cabinet Secretary was carrying out his duties in full and I was seeing him everyday chairing the meetings via Zoom.’
Asked directly if Sir Mark had coronavirus at the same time as the PM, the spokesman said: ‘He worked from home as usual throughout.
‘You will remember at the time most people were working from home so there is nothing unusual about that.’
The spokesman said he was unaware exactly when Sir Mark had developed coronavirus symptoms.
The fact that a handful of people at the top of the government caught coronavirus has previously prompted intense scrutiny of its handling of the outbreak.
Ministers have always dismissed accusations that it showed they had taken the wrong approach, insisting that it simply demonstrated the indiscriminate nature of the disease.
The PM’s top aide Dominic Cummings also developed symptoms of coronavirus, with his wife saying that he was so ill with coronavirus that she believed he should have gone to hospital
It was revealed on March 27 that Mr Johnson had tested positive for coronavirus with Mr Hancock announcing later on the same day that he too had tested positive.
Mr Johnson was then admitted to hospital on April 5 and moved to an intensive care unit on April 7 before being released on April 12.
He spent a number of weeks recovering at Chequers before returning to work on April 27.
Mr Johnson said of his illness that there were ’48 hours when things could have gone either way’.
Mr Hancock spent a week in self-isolation after his positive test. He said it had felt ‘like having glass in my throat’ and that he had lost half a stone in just seven days.
Meanwhile, Mr Cummings’ wife revealed last month that her husband was so ill with coronavirus that she believed he should have gone to hospital.
Journalist Mary Wakefield revealed that Mr Cummings, 48, spent 10 days bedridden with the disease after coming home to nurse her through a milder case.