Ahead of the hearing, Mr Johnson released a 52-page dossier of written evidence outlining his defence, while a new cache of documents released on Wednesday morning revealed fresh evidence submitted to the committee.
How to watch a Boris Johnson hearing live stream
The Privileges Committee hearing is being aired to the public, with a live stream available on this page once Mr Johnson begins giving evidence.
He is set to appear from around 2pm on Wednesday 22 March, and the session could last up to five hours.
What is the Privileges Committee?
The Privileges Committee is a Commons committee that has been charged by MPs with undertaking the parliamentary investigation into whether Mr Johnson lied over Partygate.
It is a seven-member cross-party body with a Labour chair – the grandee Harriet Harman, is the longest-standing female MP – but a Tory majority.
Other members are Tories Sir Bernard Jenkin, Sir Charles Walker, Andy Carter and Alberto Costa, Labour’s Yvonne Fovargue and the SNP’s Allan Dorans.
The committee is examining evidence from at least four occasions when Mr Johnson may have misled MPs with his assurances to the Commons that lockdown rules were followed.
It will publish its findings on whether the former Prime Minister committed a contempt of Parliament and can make a recommendation on any punishment. But the ultimate decision will lie with the full House of Commons.
What can you expect from the Boris Johnson hearing?
The committee has been clear that the purpose of the inquiry is to consider whether Mr Johnson misled Parliament, rather than comb over the various details of the Partygate scandal.
It has said: “The evidence strongly suggests that breaches of guidance would have been obvious to Mr Johnson at the time he was at the gatherings.”
The ex-PM has always protested his innocence, rejecting any suggestion that he “knowingly or recklessly misled Parliament”.
He set out his core defence in a 52-page dossier, in which he stressed that he trusted the assurances of key aides and said that “hindsight is a wonderful thing”.
It is a defence that refers to the “cramped” workplace of No 10 as well as his own belief that no guidance or rules were being breached at any gathering.
Mr Johnson placed great stock in both the assurances he had received as prime minister and the fact that no-one around had expressed concerns themselves, while also making much of the fact that there is no evidence that he ever received warnings about breaches of guidance.
If the committee finds a contempt has been committed, it will recommend a punishment which would then have to be approved by the House of Commons as a whole.
Sanctions could range from a simple apology to ordering that Mr Johnson be suspended from Parliament. Any suspension of 10 sitting days or more could trigger a tricky recall by-election in his Uxbridge and South Ruislip seat.
If 10 per cent of eligible registered voters in the constituency sign a recall petition then a by-election will be called.
Mr Johnson would be eligible to stand again, but in 2019 he had a majority of 7,210 over Labour and the Opposition would be keen to claim a high-profile scalp.
Additional reporting from Press Association