He was expected to give evidence to the Covid inquiry’s current sessions, looking at core decision-making and political governance during the pandemic, but his appearance was delayed because of illness.
Now, others set to give evidence in the coming weeks have been sent a letter suggesting his evidence session will be further delayed due to the sickness – details of which have been kept private.
A source close to the inquiry stressed that Mr Case could still be called at later sessions to the inquiry, which is expected to run for years to come.
Mr Case has been at the heart of much of the testimony heard by the inquiry so far.
The cabinet secretary, who was appointed to the top civil service job by Boris Johnson in September 2020, was part of a core group around the former PM.
The inquiry has focused on WhatsApp messages between Mr Case and other key advisers including Dominic Cummings and Mr Johnson’s former comms chief Lee Cain.
Messages sent between the three revealed the chaos at the heart of Downing Street during the pandemic, with Mr Case at one point declaring: “I am at the end of my tether.”
The head of the civil service said the government was looking like a “terrible, tragic joke”, while Mr Johnson’s wife Carrie was “the real person in charge”.
Mr Case wrote that he was “not sure I can cope” amid apparent frustration at how the pandemic was being handled in government.
And in a stunning exchange seen by inquiry, Mr Case wrote: “I was always told that Dom [Dominic Cummings] was the secret PM. How wrong they are. I look forward to telling select cttee tomorrow – ‘oh, f*** no, don’t worry about Dom, the real person in charge is Carrie’.”
Mr Case also played a key role in the Partygate saga and had to recuse himself from an investigation into lockdown-busting Downing Street gatherings due to his own involvement, handing the task to Sue Gray.
He was pictured at the Cabinet Room birthday party for which Mr Johnson and Mr Sunak were fined.
Mr Case’s central role during the Johnson administration also saw him drawn into the rows surrounding Richard Sharp’s appointment as BBC chairman.
He also admitted he had an “informal conversation” about potential roles at a royal charity for Mrs Johnson after a prompt from No 10.
On Tuesday the Covid inquiry said: “On 20 November 2023 the Solicitor to the Inquiry conveyed to Core Participants by letter private medical information regarding the Cabinet Secretary Simon Case in order to update them on his ability to give evidence during the Module 2 hearing and seek any representations which they wished to make.”
Witnesses are announced on Thursday for the following week. The inquiry declined to comment on who would appear in future weeks.
The prime minister has wished Mr Case a speedy recovery.