When Boris Johnson is hauled before MPs to account for his denials during Partygate, there will be more at stake than a potential comeback for the former prime minister. What will unfold is a tussle for power between the country’s defenestrated premier and its parliament, the likes of which, Johnson’s supporters say, has not been seen since King Charles I was summoned to Westminster Hall in 1649 and executed. In what was dubbed “the trial of the millennium”, the monarch was tried for tyranny and treason, having sidelined parliament and sparked the English civil war. The charge against Johnson is a serious breach of parliamentary privilege – by misleading MPs about law-breaking parties during the Covid pandemic lockdowns. Many of the facts are no longer in doubt, thanks to journalistic endeavour, an investigation by the senior civil servant Sue Gray and Scotland Yard’s Operation Hillman, which handed out more than 100 fines – including to Johnson and his wife. Uncovered were 12 illegal gatherings – fuelled by suitcases of wine, a DJ and a karaoke machine – in defiance of tough restrictions on socialising, while millions of people were told to stay at home and were unable to say their final goodbyes to dying relatives. What is being probed by the seven-member privileges committee, with a Conservative majority but chaired by Labour’s Harriet Harman, is less to do with what happened. Instead, the MPs want to know: who knew what, when? They gathered for their first meeting of the year on Wednesday to discuss progress, with a decision expected next week on when to launch the next – and potentially most explosive – phase of their investigation. Witnesses’ summons will be drafted, with oral evidence sessions held and a final report compiled – all hoped to be completed within two months. The inquiry is seeking to discover whether Johnson misled MPs by repeatedly denying – on the floor of the House of Commons – that any Covid laws were broken in No 10, given these denials turned out to be patently untrue. A litany of documents was demanded by the committee that Liz Truss’s government was initially reluctant to hand over. They redacted, for instance, some evidence that was publicly available in the Sue Gray report. When Rishi Sunak came to power, the new minister responsible for liaising with the committee – the paymaster general in the Cabinet Office, Jeremy Quin – acquiesced. “They’ve finally given us everything we asked for,” an insider crowed at the end of November, four months on from the initial request. Other delays compounded the inquiry’s progress. One of the committee’s Tory MPs, Laura Farris, stood down. Despite Johnson trying to install the veteran objectionist Christopher Chope on the committee, outcry over the move meant the search continued. Another longstanding backbencher, Charles Walker, was eventually chosen – but could not be formally nominated until normal parliamentary business resumed after the death of the Queen. Since mid-October, the MPs have met at least once a week, usually on a Wednesday, in a dusty parliamentary committee room, well away from prying eyes, to comb through the written evidence. Harman, a king’s counsel and former solicitor general, is a stickler for process and prefers for submissions to be packaged into large, lawyerly bundles. MPs on the committee are expected to give those they want to call for evidence – including Johnson – two weeks’ notice, with a run of hearings likely to begin in early February and lasting for up to three weeks. Johnson and other witnesses will be allowed to bring legal counsel to hearings and consult them throughout the session, but must answer themselves. The former prime minister has already been given legal support by the government, at a cost to the taxpayer of up to £220,000. Hearings were meant to start two months ago, so the fee has nearly doubled. Two bids have been launched to discredit the inquiry. The first was an opinion co-authored by the cross-bench peer David Pannick. He argued that the investigation was “fundamentally flawed” because it neglected to require proof that any misleading of parliament by Johnson was intentional. Pannick warned this would have a “chilling effect” on all statements by ministers at the dispatch box. The Guardian has been told that the advice was leaked as a “PR exercise” by a government special adviser. MPs on the committee hit back, arguing Pannick’s criticisms were based on a fundamental misunderstanding of the process, given that the inquiry is a parliamentary one rather than judicial – and subject to different standards than a criminal investigation. A second counteroffensive is also under way by Johnson’s most diehard political supporters to paint the inquiry as a “witch-hunt”. Their focus is less on the remaining stages of the investigation – the hearings and preparation of a final report – and instead, the endgame. The privileges committee could absolve Johnson, or find against him and suggest a sanction that theoretically could be severe enough to see him face the threat of a byelection. However, it can only make a recommendation about what happens next. Everything will come down to a vote in the Commons, likely to be officially unwhipped given it is “House business”. If the committee believes Johnson did mislead parliament, each Tory MP must decide for themselves whether to absolve him or again betray the leader who won them a hefty majority in 2019. To substantiate conclusions reached in the final report, evidence – including pictures and text messages – could be included. Johnson’s allies are prepared to argue that it runs contrary to “natural justice” to punish him further, given he has already been removed from No 10. But renewed whispers about the possibility of a Johnson comeback is sharpening minds. Few Tory MPs may be prepared to let him off the hook so he can shrink into the shadows with some dignity, only for him to return – like the banished son of Charles I who managed to re-take the crown.