Sue Gray is unlikely to produce the conclusion that many are hoping for. She should not be expected to do so. Her task is to discover the facts, not pronounce a verdict. The first issue on which she is asked to pronounce is whether the Downing Street parties broke either government guidance or the law, an offence for which many were fined. That, however, should not be a matter for her but for the police. The Met gave two feeble excuses for not investigating. The first was that the parties were held long ago: as if a crime committed 18 months ago is exempt from investigation – good news for criminals who can cover their tracks. The second excuse was insufficient evidence. But where it seems that an offence has been committed, it is the Met’s job to search for evidence, which indeed was right in front of their eyes since police are present in Downing Street at all times and will have seen people coming and going. Gray, by contrast, has no professional qualifications as a detective or powers to compel the production of evidence, and so must rely on what is said to her, as Lord Geidt had to do in his inquiry into the furnishing of the Downing Street flat. In any case, Gray may be impartial but she is not independent. A former second secretary in the Cabinet Office, she is now a permanent secretary. She reports to the prime minister, who might well not look favourably on her promotion prospects if the report condemns him and he remains in office. Her boss is Simon Case, the cabinet secretary and head of the civil service, one of whose responsibilities is to evaluate her performance. It is, moreover, never right to rely upon one investigator, however eminent. A stronger constitutional check is needed on ministers and the prime minister – a permanent investigatory panel of three wholly independent people from outside the civil service, at least one of whom should be legally qualified. If the report turns out to be unfair to the prime minister, he has redress in parliament. Officials criticised in the report have no such redress. They cannot defend themselves in public and their careers and reputations could be severely damaged, if not ruined. The second issue on which Gray is to pronounce is whether Johnson deliberately deceived parliament. The ministerial code decrees that any minister guilty of this offence must offer their resignation to the prime minister. There is, however, no mechanism to enforce this rule against ministers, still less against the prime minister, and a non-elected official such as Gray is unlikely to take it upon herself to tell an elected prime minister that he must go. In theory, the prime minister could be prosecuted for the old common law offence of misconduct in public office. But there appears to have been no such prosecution of a politician since the early 19th century, and the courts would hardly intervene in what is, in the last resort, a political issue. For similar reasons, the weapon of impeachment has long fallen into a state of disuse. Johnson would not have been the first occupant of No 10 to have intentionally misled parliament. There is the case of Anthony Eden who, with his foreign secretary, Selwyn Lloyd, misled the Commons by denying collusion with Israel in the Suez invasion in 1956. They believed that otherwise the lives of British residents in Arab countries might be endangered. That the house did not hold it against them is shown by the fact that Lloyd was elected Speaker in 1971. In the days of fixed exchange rates, chancellors were regularly excused for not announcing that a government had decided to devalue the pound. John Profumo, however, was not forgiven in 1963 for misleading the house concerning his relationship with Christine Keeler. There is, then, no fail-safe mechanism to enforce the constitutional convention embodied in the ministerial code. It could in theory be enforced by a Commons vote of no confidence. More likely, however, is a vote of no confidence by Conservative MPs in a secret ballot. In normal times, ministers can rely on a solid phalanx of parliamentary support; but these may not be normal times. Scottish Conservative MPs appear to be particularly opposed to the prime minister, and there appears to be a growing rift between them and English Conservatives, a rift that could endanger the very union Conservatives are pledged to defend. But constitutional conventions depend in the last resort upon public feeling. Voters may conclude that the parties are of no real significance; or they may feel deep and long-lasting anger against those in authority who broke rules they themselves made and which the public observed, often at the cost of great stress and anguish. In that case they will tell Conservative MPs that Boris Johnson must go. It is not Sue Gray, or even Conservative MPs, who will decide the prime minister’s fate, but we the people. Vernon Bogdanor is professor of government, King’s College London. His books include Beyond Brexit: Towards a British Constitution