Former British Prime Minister Boris Johnson is stepping down as a member of parliament with immediate effect, triggering a by-election in his marginal seat.

Johnson had been fighting for his political future with a parliamentary inquiry investigating whether he misled the House of Commons when he said all COVID-19 rules were followed.

"It is very sad to be leaving parliament - at least for now," Johnson said in a statement. "I am being forced out by a tiny handful of people, with no evidence to back up their assertions, and without approval even of Conservative party members let alone the wider electorate."

Parliament's privileges committee could have recommended that Johnson be suspended from parliament for more than 10 days if they were to find he did mislead parliament recklessly or deliberately, potentially triggering an election for his seat.

The former prime minister said he had received a letter from the "privileges committee making it clear - much to my amazement - that they are determined to use the proceedings against me to drive me out of parliament".

Johnson, whose premiership was cut short in part by anger in his own party and across Britain over COVID rule-breaking lockdown parties in his Downing Street office and residence, accused the committee of acting of being the "very definition of a kangaroo court".

"Most members of the Committee - especially the chair - had already expressed deeply prejudicial remarks about my guilt before they had even seen the evidence," he said.

"In retrospect it was naive and trusting of me to think that these proceedings could be remotely useful or fair."