Boris Johnson has been given the findings of an investigation into whether he misled parliament over Partygate, the Guardian has been told. With the nearly year-long inquiry drawing to a close, sources said a “warning letter” had been submitted to the former prime minister containing criticisms for him to respond to. At stake is the future of Johnson’s political career – a suspension of 10 days or more could lead to a byelection being triggered in his Uxbridge and South Ruislip constituency. Under the “Maxwellisation process”, Johnson was handed a dossier laying out the committee’s findings. It is said to contain the facts and evidence they are based on and any proposed sanctions. Johnson will have two weeks to reply, with the whole process conducted in secrecy. His response will be considered by the privileges committee before it finalises its report, expected to happen by the end of June. MPs then have to endorse the findings, which could threaten to shatter the fragile peace in the Conservative party that Rishi Sunak and his allies have attempted to broker. The government will get to choose when to table the motion to endorse the committee’s report. It is expected to come before the Commons summer break, which starts on 20 July. MPs are likely to be given a “free vote” – though sometimes informal pressure is applied by telling them which way the chief whip is going to vote. MPs will also be able to amend the motion. Dozens of Tory backbenchers recently voted against a finding by the standards committee that another MP, Margaret Ferrier, be suspended from parliament for 30 days, for breaking Covid rules. Some have sought to draw parallels between the case and Johnson’s, but insiders played down the likelihood that the action taken against Ferrier set a precedent for potential punishment facing the former prime minister. All MPs passed a motion that set up the inquiry in April 2022, which examined whether he had misled parliament by repeatedly denying any Covid rules had been broken. Scotland Yard has issued more than 120 fines over Partygate – including to Johnson himself. A spokesperson for the privileges committee, which has a Tory majority but is chaired by Labour’s Harriet Harman, said it was “proceeding in accordance with its previously published procedure”. They added: “If the committee decides to criticise Johnson, it will not come to a final conclusion until it has taken into account any further submissions from Johnson.” Johnson’s spokesperson was contacted for comment. They have previously said he was cooperating fully with the committee and insisted he “did not knowingly, deliberately or wittingly mislead parliament”. Relations between Johnson and Sunak have deteriorated in recent weeks, after government officials handed over a tranche of his diaries from the pandemic prompting further investigation by two police forces and the privileges committee. In a move that has caused a major headache for Sunak, Johnson then handed over a batch of unredacted files, including WhatsApps, to the Covid inquiry – despite the Cabinet Office taking legal action to avoid doing so. Fresh details about an event at Chequers during Covid were also revealed this week by the Guardian. But the privileges committee is not likely to hold up its final report to investigate them, as several such details were said to have been identified as part of the diaries handed to it last month. Though Johnson’s allies have been wargaming how to avoid him being forced out of parliament by a negative finding by the privileges committee, public opinion appears to be against him. More than two-thirds of people (68%) believe he should resign as an MP if found to have intentionally misled parliament, according to a new survey by YouGov. The poll found that 56% think he lied about breaching lockdown rules and it was right for him to be held accountable, while a further 29% believe he lied but that the issue should be moved on from. Just 5% believe that Johnson did not lie and there was nothing to hold him accountable for, in a survey of 2,071 adults conducted on 6 and 7 June. Despite friends of Johnson, such as the former culture secretary Nadine Dorries, trying to paint the inquiry as a “witch-hunt”, 60% of Britons thought the inquiry was fair compared with 21% who said it was unfair.