The Christian Institute is threatening to take legal action against the Government if a proposed ban on conversion therapy ends up criminalising the "wrong kind of prayer".
The Government has promised to ban conversion therapy, a move that has been supported by liberal campaigners like Jayne Ozanne, who has claimed that biblical Christian teaching on human sexuality is a safeguarding issue.
Evangelical Christians fear that if conversion therapy is banned, pastors could face criminal charges simply for praying with or counselling someone who struggles with their sexual identity.
Jason Coppel QC, in a detailed legal opinion for the Christian Institute, believes that a broad definition of the law will criminalise the ordinary work of churches, including prayer, evangelism, baptism and communion.
There are implications for parents too, as any requirement to affirm their child's chosen gender identity would endanger their ability to bring up their children in accordance with their beliefs, Mr Coppel warns.
If the legislation is similar to that recently passed in Victoria, Australia, Mr Coppel says this would be a breach of UK human rights law protecting "the freedom of church organisations to preach", and would in effect "require conformity" around beliefs on sexual ethics and gender identity.
The legal opinion says that the courts have "consistently regarded such beliefs as protected by Article 9 ECHR and worthy of respect," and that it would be "particularly difficult to justify" the criminalisation of traditional beliefs on sexuality and gender identity.
He argues that such beliefs are "not eccentric or peripheral", and that the state should remain neutral and impartial.
The Christian Institute's Deputy Director for Public Affairs Simon Calvert, said: "A ban on spiritual guidance and prayer would be tyrannical and unworkable. Do they expect police, prosecutors and courts to decide which kinds of prayer are criminal and which are not?
"Most people would be horrified by the prospect of someone being convicted for praying 'the wrong kind of prayer'. We must not allow activists to exploit legitimate concerns as a cover for pursuing anti-religious agendas."
He continued: "Our lawyers have today made clear to the Equalities Minister that we are prepared to seek judicial review if the Government caves into demands to widen the ban to include the ordinary, everyday activities of churches.
"If they were to introduce such a ban, we are confident a court would find it to be a breach of human rights law."