Nadine Dorries has accused to committee of MPs investigating whether Boris Johnson lied to parliament of conducting a “witch hunt”.
The prime minister is being investigated by the Commons privileges committee over whether he misled parliament during the partygate affair.
Johnson has always said he did not knowingly lie when he said no Covid rules were broken in Downing Street.
But the committee, led by veteran Labour MP Harriet Harman, will simply rule on whether the prime minister misled the House, intentionally or unintentionally.
If he is found guilty, Johnson could face a by-election in his Uxbridge and South Ruislip seat which might see him voted out of parliament.
Dorries, Johnson’s closest cabinet ally, tweeted on Sunday: “If this witch hunt continues, it will be the most egregious abuse of power witnessed in Westminster.
“It will cast serious doubt not only on the reputation of individual MPs sitting on the committee, but on the processes of parliament and democracy itself.”
Labour MP Chris Bryant replied: “Let’s talk about abuse of power such as illegally suspending parliament or doling out peerages to donors or tearing up the rules to protect Owen Paterson.
“The real abuse of power would be suspending an inquiry to protect your mate.”
Bryant is the usual chair of the privileges committee, but stood aside for the partygate inquiry given his past criticisms of Johnson.
The committee intends to call the PM to give oral evidence in public in the autumn, under oath.
The committee has already said that whistle-blowers will be able to give evidence about the prime minister anonymously.
Johnson has also been ordered to hand over a cache of documents to the MPs, including diaries, photographs and deleted documents.