Boris Johnson deliberately misled MPs over Covid lockdown-breaking parties in Downing Street when he was prime minister, a UK parliament committee ruled on Thursday.

The cross-party Privileges Committee said Johnson, 58, would have been suspended as an MP for 90 days for "repeated contempts (of parliament) and for seeking to undermine the parliamentary process".

But he avoided any formal sanction by his peers in the House of Commons by resigning as an MP last week.

In his resignation statement last Friday, Johnson pre-empted publication of the committee's conclusions, claiming a political stitch-up, even though the body has a majority from his own party.

He was unrepentant again on Thursday, accusing the committee of being "anti-democratic... to bring about what is intended to be the final knife-thrust in a protracted political assassination".

Calling it "beneath contempt", he said it was "for the people of this to decide who sits in parliament, not Harriet Harman", the veteran opposition Labour MP who chaired the seven-person committee.

The committee's long-awaited 106-page report was even more critical than expected, particularly in relation to the sanction it would have recommended.

MPs ordinarily have to vote on the committee's recommendations, with any suspension over 10 days potentially triggering a "recall" by-election in the offender's constituency.

Johnson hung on to his Uxbridge and South Ruislip seat in outer northwest London at the last general election in December 2019, at which his Conservative party won a landslide.

But with only a 7,200-seat majority and with the Tories well down against Labour in the polls, it would have been no means certain that he could have won.

The "Partygate" scandal saw Johnson and dozens of government officials fined by police for breaking the social distancing laws that the government set the public to curtail the spread of Covid-19.

It triggered public outrage, particularly among the families of those who died from the virus.

The scandal was one of a number that contributed to Johnson's downfall as prime minister, and led to a ministerial rebellion that forced him to resign last July.