Former UK prime minister Boris Johnson on Wednesday angrily denied he lied to Britain's parliament over the "Partygate" scandal as MPs held an inquisition that could decide his political fate.

Opening the crunch hearing by a House of Commons committee, the ex-Conservative leader swore an oath on the Bible and declared that "hand on heart, I did not lie to the House".

Johnson said it would have been "utterly insane" to lie about the gatherings in 10 Downing Street, many of which were found by police to have broken pandemic lockdown legislation that he introduced.

"People who say that we were partying in lockdown, simply do not know what they are talking about," Johnson added, insisting that some booze-fuelled gatherings should have been viewed as workplace meetings.

Privileges committee chairwoman Harriet Harman said the hearing went to the "heart of our democracy", because if MPs are wilfully misled by ministers, "we can't do our job".

Harman is a veteran member of the Labour opposition party, dubbed the "Mother of the House" by dint of being the longest-serving woman in the Commons.

The rest of the seven-strong committee has a majority drawn from Johnson's Conservative party – and they proved to be his toughest interrogators.

The hearing confirmed that Johnson had relied on assurances given by political aides, rather than senior civil servants or government lawyers, when he told the House that all lockdown rules were followed.

MPs were entitled to feel "a bit dismayed about the flimsy nature of this assurance", Harman told Johnson, who grew increasingly testy towards the hearing's conclusion.

But rejecting the contention of Johnson loyalists that the hearing amounts to a "kangaroo court", Harman said the MPs were acting only "in the interests of the House".

Johnson disavowed such language from his supporters, but insisted there was "not a shred of evidence" to find him guilty of contempt towards the House of Commons.

Supporters insist Johnson was betrayed by Conservative colleagues when he was forced out of office last year, and are campaigning for his return ahead of a general election likely next year.

But opinion polls suggest that Johnson remains toxic for a large swathe of the electorate, and the committee hearing reopened old wounds just as Prime Minister Rishi Sunak tries to revive Conservative fortunes.

Johnson's diminishing hold over the party was underlined even during the hearing, as the MPs paused to join a Commons vote on a post-Brexit deal agreed by Sunak with the European Union concerning Northern Ireland.

Johnson, pro-UK unionists from Northern Ireland and a number of Tory Brexit hardliners voted against a key part of the deal.

But Sunak won an emphatic victory, by 515 votes to 29.

If the committee decides that Johnson lied to parliament about the partying, it could recommend his suspension from the House of Commons.

If the full House agrees to a suspension of more than 10 sitting days, that could trigger a special election for his northwest London seat, if enough voters demand one.

Hours before Wednesday's televised hearing, it published a larger 110-page bundle of evidence.

It included a Downing Street official stating that Johnson "often saw and joined" gatherings in the complex during lockdowns, and that "he had the opportunity to shut them down".

"He could see what was happening and allowed the culture to continue," the official added.

On Tuesday, Johnson released his own 52-page dossier detailing his belief that he was truthful when he repeatedly told parliament that all regulations were respected.

In hindsight, he recognised that he did "mislead" MPs, but only inadvertently and based on assurances given by top aides that the rules were being followed.

Johnson was fined by police for one gathering, along with Sunak, his finance minister at the time. Dozens of other staff members also received fines.

The former leader apologised and corrected the parliamentary record last May after previously insisting to MPs that the gatherings were above board.

Johnson said that correction came at the earliest opportunity – after London police and senior civil servant Sue Gray had concluded their own investigations.

Johnson, 58, nearly died himself of Covid. But the campaign group Covid-19 Bereaved Families for Justice UK said his claim that he issued his Partygate denials "in good faith" was "sickening." (AFP)