MPs investigating Boris Johnson over Partygate are drawing up a rebuttal dossier to criticisms made by a legal expert about the process, as they push to avoid the inquiry being “derailed”. The privileges committee will issue a staunch defence of its work after being admonished by Lord Pannick, a crossbench peer and QC who claimed it was “proposing to adopt an unfair procedure”. Nearly £130,000 of taxpayers’ money was spent on legal advice commissioned by the Cabinet Office in the dying days of Johnson’s administration that sought to discredit the committee’s work. The advice prompted fresh claims from the former prime minister’s allies that he was the victim of a “witch-hunt”. In response, a “special report” is being drafted that will be published as soon as Thursday by the committee examining claims that Johnson falsely told the Commons he knew nothing about lockdown-breaking gatherings. It is expected to justify the collection of evidence from some anonymous whistleblowers – such as longstanding civil servants – and dispute Pannick’s suggestion that not needing to prove the denials of wrongdoing were intentional would be “unlawful”, if tested in court. Pannick’s claim that punishing ministers for making such mistakes on the floor of the House would have a “chilling effect” will also be addressed, with a reminder of the procedures available to MPs who make an error and want to correct the record. The Guardian also understands that the committee will send a letter to Truss’s new government, asking if it shares Pannick’s views, and if so, challenging ministers to pass a new motion to change the rules about how the inquiry should be run. At their first meeting since Johnson stepped down as prime minister on Wednesday, MPs on the committee agreed to continue trawling through written evidence they received over the summer despite being one member short. They want the slot filled before the conference recess running from 22 September to 17 October, after which they hope to hold oral evidence sessions. Tory backbencher Laura Farris quit in August, but the colleague Johnson lined up to replace her, the veteran objector Christopher Chope, was not endorsed by the new Liz Truss administration. Chope’s nomination was contingent on being approved by the Commons. If passed, it would have restored the Conservatives’ majority on the committee. Though there was significant resistance to Chope’s nomination among opposition parties, sources said some on the committee feared the investigation could be “derailed” or plunged into stasis if he was blocked from joining. The six current members are: Labour’s Harriet Harman, who is chairing the inquiry, and Yvonne Fovargue; Alberto Costa, Andy Carter and Bernard Jenkin from the Conservatives; and the SNP’s Allan Dorans. The MPs are meeting again on Thursday, and could sign off on the publication of their special report that same day. After this week’s cabinet reshuffle, a new chief whip, Wendy Morton, will face the decision of which Tory backbencher to put forward to join the committee. Asked why the government did not push ahead with the nomination of Chope, Truss’s press secretary said on Wednesday: “There’s a new chief whip – and she needs to be given time to consider the matter.” They added: “I can’t put a timetable on it.” Johnson still faces investigation, even though he has now departed Downing Street. Evidence requested from No 10 so far includes WhatsApp messages, photos and diary entries. When they complete their investigation, MPs on the privileges committee can recommend a punishment for those found to have broken Commons rules. Potential sanctions include publishing a written apology, as well as suspension or expulsion from parliament. They must be endorsed by the Commons to be enforced.