It was reported at the time that he was likely to be off for a number of weeks and that he would be returning in 2024.
A spokesperson for the Cabinet Office told Sky News: “The cabinet secretary will return to work in the new year.”
It is understood that when he returns to the civil service, he will resume the same responsibilities in his capacity as cabinet secretary.
The Cabinet Office would not comment on the medical advice he has received as it is a private matter.
There have been questions surrounding Mr Case’s return to work after the Times reported his initial four-week leave of absence in October had been extended to 2024, generating concerns about a vacuum at the heart of government.
Mr Case, who was appointed in September 2020 having previously served as permanent secretary in Number 10, was initially due to give evidence to the official COVID inquiry this year.
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But inquiry chair Baroness Heather Hallett formally excused him from doing so “due to ill health”.
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1:29
Johnson asked about ‘let virus rip’ comment
In a ruling last month, Baroness Hallett said she would receive an update on Mr Case’s ability to give evidence at the end of January 2024 or on his return to work from sick leave, whichever is sooner.
The COVID inquiry is looking into the UK’s handling of the pandemic, including government decision-making around controversial areas such as imposing three national lockdowns.
Although the inquiry has now finished hearing evidence under module 2, Baroness Hallett said it “very much remains my intention that Mr Case should give oral evidence to the inquiry” and that she will convene a special hearing for module 2 to receive his evidence.
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2:10
Rishi Sunak appears at the COVID inquiry
Despite his absence, Mr Case has featured prominently in the evidence heard at the inquiry so far via WhatsApp messages exchanged between senior figures during the pandemic.
Messages understood to be from 14 October 2020 were disclosed to the inquiry, in which Mr Case described Mr Johnson’s wife, Carrie Johnson, as “the real person in charge”.
Mr Case also said the government was looking like a “terrible, tragic joke” and that he “cannot cope with this” in an apparent expression of frustration at how the coronaviruspandemic was being handled.
In another fractious message shown to the inquiry in October, Mr Case described “being at the end of my tether” over Mr Johnson’s indecision during the pandemic and said he “cannot lead”.
When approached for a comment by Sky News, the COVID inquiry said it does not provide a running commentary on the progress of its investigations.