Boris Johnson would have “contaminated” the government with lies had he not been investigated for his Partygate denials, MPs have been told, as they debated sanctioning him for committing contempt of parliament. In a series of scorching speeches about the former prime minister’s conduct, Conservative and opposition MPs said Johnson deserved to be censured for misleading the Commons. Harriet Harman, the chair of the privileges committee, used her first public comments about the inquiry she has chaired for nearly 14 months since its report into Johnson was published last week to say the matter “could hardly have been of more importance”. She said: “Because he was prime minister, Johnson’s dishonesty – if left unchecked – would have contaminated the whole of government, allowing misleading to become commonplace, and thus erode the standards which are essential for the health of our democracy.” Harman addressed criticism by Jacob Rees-Mogg, an ardent supporter of Johnson, who had questioned whether the inquiry looked biased given some of her previous tweets about his conduct. She said she had offered to stand down, but had been told the government wanted her “to continue”. Tory MPs on the committee had “had to be extraordinarily resilient”, Harman added. She said they had endured a “campaign of threats, intimidation, and harassment designed to challenge the legitimacy of the inquiry, to drive them off the committee and thereby frustrate the intention of the house that this inquiry should be carried out”. The Commons leader, Penny Mordaunt, cut a lone figure on the government benches, but was defiant about her belief that Johnson had misled parliament and should be punished. She said she would be voting to approve the report, which was debated by MPs for around four hours on Monday evening, but added: “All members need to make up their own minds and others should leave them alone to do so.” Mordaunt said there were “meaningful consequences” to the privileges committee’s report, as the MPs on the cross-party group had “sought to defend our rights and privileges in this place … not to be misled”. Theresa May kept up the pressure on Johnson, saying MPs were “responsible for our own actions” and that friendship between colleagues should not cloud their judgment. “It is doubly important for us to show that we are prepared to act when one of our own, however senior, is found wanting,” added May. “Following an unsettling period in our political life, support for the report of the privileges committee will be a small but important step in restoring people’s trust in members of this house and of parliament.” MPs are expected to vote on the report on Monday evening, with Johnson’s supporters boycotting it. Some of them repeated their attacks on what they called a “kangaroo court”.