The Scottish Government has been warned it could face a courtroom challenge if it brings in what campaigners have claimed would be the “most totalitarian conversion therapy ban in the world”.

Simon Calvert, a deputy director at the Christian Institute, said his organisation was preparing the ground for legal action, if Holyrood ministers attempted to change the law.

It comes amid fears that legislation could end up criminalising the “mainstream pastoral work of churches, mosques and synagogues and temples”, as well as parents.

In a legal opinion obtained by the Christian Institute, lawyer Aidan O’Neill KC said the recommendations made by a Scottish Government-appointed expert group “undoubtedly involve proposals for radical changes in the current law and a marked expansion in the powers of the State”.

The recommendations were “fundamentally illiberal in intent”, the lawyer added.

It comes after the expert group report, published in October last year, argued that conversion therapy could “infringe upon the human rights of individuals, in particular the victim’s freedom from discrimination and freedom from non-consensual medical treatment”.

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It recommended the criminalisation of such activity – saying this should not only apply to those carrying out conversion practices, but also those promoting and advertising them, and those instructing others to carry out such behaviour.

Removing someone from Scotland to be subject to conversion practices anywhere else in the world should also be considered a criminal offence, the report added.

Ministers established the group after the Scottish Government’s 2021/22 programme pledged to “protect LGBT people from harm by banning the damaging promotion and practice of conversion therapy, bringing forward legislation that is as comprehensive as possible within devolved powers by the end of 2023, if UK Government proposals do not go far enough”.

But Mr Calvert insisted: “If the Scottish Government follows the advice of its expert advisory group it will be exceeding its powers and inflicting the most totalitarian conversion therapy ban in the world.”

Ministers were warned they could face a protest similar to the one against the “named person” policy, a controversial measure dropped after a legal challenge which had sought to appoint a single named person to safeguard the welfare of every child in the country.

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Mr Calvert added: “Church workers, feminist activists, mums and dads – all sorts of innocent people could find themselves on the wrong end of a prosecution if this becomes law.

“And I think Scottish taxpayers will eventually find themselves picking up the legal bill for another court defeat.”

He spoke out after the legal opinion given by Mr O’Neill suggested the “apparent intent” of the expert group’s recommendations was “to criminalise, among other things, the open expression of such orthodox traditional religious beliefs that sexual activity is only properly permissible within the bounds of an opposite-sex marriage, if said with a view to encouraging another to refrain from same-sex sexual behaviour”.

The lawyer added: “The aim of the expert group’s proposals is to outlaw all and any religious pastoral care, or parental guidance, or advice or medical or other professional intervention relating to sexual orientation, expression of sexual orientation, gender identity and/or gender expression except that which is deemed by the State to constitute ‘affirmative care’.”

He continued: “Were the Scottish Government to adopt the expert group’s recommendations and legislation were introduced and passed to give effect to them by the Scottish Parliament, this would have the undoubted effect of criminalising much mainstream pastoral work of churches, mosques and synagogues and temples.”

Mr O’Neill went on to state that, if enacted, the recommendations could also “criminalise medical practitioners who express a professional opinion that it may not be in a patient’s best medical interest to undergo or undertake gender reassignment” – as well as parents who “lovingly and in good faith” sought to “caution” their child over gender transition.

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He concluded: “In my view, it would be beyond the powers of the Scottish Parliament to legislate along the terms recommended by the expert group.”

Mr Calvert said: “LGBT people are rightly protected from physical and verbal abuse by existing law just like anyone else. But these proposals go much, much further.

“The Scottish Government is considering a law that could criminalise churches and gender-critical feminists alike simply because their conversations around sex and gender don’t conform to a narrow, state-approved brand of LGBT politics.”

He continued: “We’re very concerned that the expert advisory group is urging them to pass a law that would put the ordinary work of churches in the firing line.”

Mr Calvert added: “The Scottish Government and the Scottish Parliament must take Aidan O’Neill’s advice seriously.

“They may not like what Christians have to say about sexuality, or what feminists have to say about gender identity, but they can’t just criminalise opinions they don’t like.”

The Scottish Government intends to introduce a Bill at Holyrood by the end of 2023, with a full consultation on the proposals expected to take place this year.

A Scottish Government spokesperson said: “Conversion practices that try to change a person’s sexual orientation or gender identity are harmful, discriminatory, and have no place in our society. We’re committed to introducing legislation that will end these harmful practices as far as possible within devolved competence just as many jurisdictions across the world have already done, and UK Government intend to partially do.

“This will be done fully recognising and respecting the legal right to freedom of religion, expression and a private and family life, which are protected under existing laws.”