Image source, PA Media

Simon Case was due speak to the inquiry last year about his role running the civil service during the pandemic, but had to withdraw in October over health issues.

Although he has not yet given evidence in person, Case has already come up in the inquiry through published WhatsApp messages.

In messages from September 2020, the inquiry heard he told colleagues former prime minister Boris Johnson "cannot lead" and "changes strategic direction every day".

We won’t be covering today's inquiry appearance with regular text updates but if you’d like to hear what he has to say, click the play button at the top of this page.

Simon Case’s role as the prime minister’s chief adviser came under heavy scrutiny last year, as Rishi Sunak faced a series of controversies and scandals in which Case’s role was questioned.

These included the arrangement of an £800,000 credit facility to Boris Johnson and questions over what he knew about an HMRC investigation into Nadhim Zahawi's tax affairs.

To his allies, Case is considered to be a highly effective civil servant with a firm grasp on his duties.

But Case’s critics accuse him of failing to robustly challenge the PMs he has worked for.

His role in the pandemic is particularly controversial, after he had to withdraw from the initial investigation into the Partygate scandal after it emerged that an event breaching lockdown rules was hosted in his office.

Read more about why Case’s role has come under the spotlight here.

Image source, Cabinet Office

Simon Case is the head of the civil service, which makes him probably the most important unelected person in politics.

The cabinet secretary’s role is to advise the prime minister, lead implementation of the government's policies and manage other high-level civil servants.

Born in Bristol in 1978, he first joined the civil service as a Ministry of Defence policy adviser in 2006.

Over the next eight years he held roles in the Cabinet Office, the Northern Ireland Office and intelligence service GCHQ.

In 2012 he became private secretary to then-Prime Minister David Cameron, going on to work in more senior roles within the Cabinet Office until being promoted to the role of Cameron’s principal private secretary.

Case went on to be part of the team involved in the Brexit talks, but left in 2018 to become Prince William's private secretary.

He made a return to Downing Street in 2020 to help with the government's coronavirus response, a role that is due to be scrutinised by the inquiry today.

Read more about Case’s pre-pandemic career here.