Parliament's privileges committee is investigating whether Johnson, who was ousted from Downing Street in September, intentionally or recklessly misled parliament in a series of statements about the rule-breaking parties.
The former leader, who considered an audacious bid for a second stint as prime minister last year, is due to give evidence to the committee on Wednesday over several hours in a televised session.
If the committee finds Johnson intentionally misled parliament then he could be suspended from the House of Commons. Any suspension longer than 10 days could prompt a by-election in his constituency.
The so-called Partygate scandal ultimately contributed to the downfall of Johnson, after months of reports that he, alongside other senior government figures, had been present at alcohol-fuelled gatherings in Downing Street during 2020 and 2021 when most of the rest of Britain were forced to stay at home.
The outcry and repeated allegations of lying eventually prompted the resignations of much of his top team of government ministers, including the current prime minister, Rishi Sunak.
In an interim report published this month, the committee - made up of seven MPs with a majority of members from the governing Conservative Party - said Johnson might have misled parliament on four occasions and said the rule breaking should have been "obvious".
On Tuesday, Johnson said in written evidence he had misled parliament but insisted he "would never have dreamed" of doing so intentionally.
Johnson described some of the allegations he faced from the committee as illogical and absurd, accusing it of being highly partisan.
In passages that are likely to form part of his defence, Johnson said there was no evidence that indicated he intentionally misled parliament and he was not warned that the events broke any rules.
The committee said Johnson had submitted his evidence late, that it had contained errors and no "new documentary evidence".
If the committee recommends any sanctions against Johnson they would have to be approved by parliament.
Sunak suggested this week that any vote on any sanctions against Johnson would be based on MPs' own beliefs rather than along party lines.
