If you want to know what is wrong with the British governing class, look no further than Labour’s Conversion Practices Bill.

The bill, now before parliament, makes it a crime, punishable by up to five years in prison, to engage in any ‘abusive conversion practice’ likely to cause severe alarm or distress. Under the bill, ‘conversion practice’ means any conduct whatsoever aimed at persuading someone against a particular sexual orientation or transgender identity. For good measure, where an organisation is involved in ‘conversion therapy’, anyone involved in running it is equally guilty if he or she does not take all reasonable steps to stop it.

It is not difficult to see why this is a terrible idea. It may outlaw practices any decent person would want banned, such as drugs or electric shock treatment to ‘cure’ homosexuals, for example. But these haven’t happened for years anyway – and besides, they are already caught by existing laws and professional prohibitions. The new bill goes much, much further.

Its vague drafting potentially threatens almost anyone who expresses strong views about the vexed issue of ‘gender identity’ with a criminal record. There is (one suspects quite intentionally) no carve-out for religious advisers, counsellors or even parents. As Sex Matters and Policy Exchange have pointed out, it could criminalise a father for confiscating black-market transition drugs and chest binders from a daughter who insists she is really a boy. Or a priest telling a parishioner his views on homosexuality and thereby causing him distress and anxiety. Or even a wife objecting to a cross-dressing husband’s behaviour. Or, for that matter, anyone ‘dead naming’ trans people or failing to use their preferred pronouns.

Of course, the government denies any intention to put parents, pastors or prelates in prison for indiscreet words. As Guernsey politician Jayne Ozanne, a fairly fanatical supporter of the bill, told the Commons select committee this week, even if it did become a crime for a priest to pray for a gay man, prosecutors can use their discretion not to pursue the case.

This is true. But that actually exposes what is seriously objectionable about this law. Essentially, it has been drafted without any regard to the concern we normally find in criminal law – namely, not to make innocuous behaviour punishable. Instead, its catch-all drafting is aimed at doing two things, both very dangerous. One is to remind the rest of us plebs to watch our step carefully when talking about sexual or gender orientation, and preferably keep quiet. The other is to hand questions of acceptable speech to a cadre of professionals and prosecutors.

Letting the lanyard class determine the boundaries of free speech is bad enough. But it gets worse. True, we may not find ourselves in court over incautiously expressed views on sexuality or gender identity. But our speech will still be effectively constrained. Even if priests praying for parishioners’ sexuality don’t have their collars felt, their churches (and mosques) will face advice from their lawyers to play it safe.

Again, suppose parents want to tell their slightly foolish 10-year-old that they don’t believe he is a girl, and that he should stop being silly. They may not face an immediate police visit, but if that child mentions his parent’s views at school, it is possible the matter will end up with a teacher’s report, and the parents in the clutches of the child protection authorities. Will other parents still want to speak their minds after that?

This is a bad bill, presented largely to keep a collection of pressure groups happy. It is unnecessary, authoritarian and a serious threat to our liberty. It needs to be decisively thrown out.

Andrew Tettenborn is a professor of commercial law and a former Cambridge admissions officer.