The Scottish Government's planned conversion therapy ban will be outwith Holyrood’s competence, a leading lawyer has warned.

In a scathing legal opinion, Aidan O’Neill KC said the measures put forward last month would mark a “disproportionate intrusion into private and family life and freedom of religion and freedom of expression.”

He said the proposals could also criminalise parents who tried to stop their teenagers from expressing their heterosexual orientation by dressing in an overly sexualised and sexually provocative and explicit way.

The opinion was commissioned by the evangelical campaign group, the Christian Institute, who have warned ministers that they will challenge any new law “all the way to the Supreme Court if necessary.”

READ MORE: Ministers 'reflecting' on conversion therapy ban concerns

The Scottish Government has insisted that they are still in “listening mode” over the ban.

In January, they launched a consultation, publishing an 86-page proposal document alongside with slew of suggestions.

The central proposal is for a new criminal offence of engaging in conversion practice “whether that is provided by a healthcare practitioner, a family member or a religious leader”.

The paper defines conversion practices as where there is “a purpose or intention to change or suppress another individual’s sexual orientation or gender identity".

It then gives several examples, including prescribing medication to suppress a person’s sex drive, or therapy or counselling that requires a person not to act on their same-sex attraction, including through celibacy.

They also suggest making it an offence to restrict where a person goes, who they see and how they dress.

Any such action would become illegal with one possible punishment being "imprisonment for a term not exceeding seven years."

In his opinion, Mr O’Neill said the SNP-Green administration had “forgotten its duty to take seriously its obligations to maintain the conditions of and for a liberal democracy, preferring instead to impose, by virtue of its possession of a monopoly on legitimate violence, its own vision of the good life.”

On the impact on family life, the KC said the proposed measures could “criminalise parents who, lovingly and in good faith and in accordance with their own best judgment and conscience, seek to caution and direct their children against acting on any stated intention to embark on ‘gender affirmatory’/‘gender transition’ treatment in respect of their currently experienced discomfort or dysphoria in relation to their sex and/or sexuality.”

He said it would also criminalise parents “seeking to control how their child ‘presents’ in terms of, say clothes, make-up, and hairstyle or imposing restrictions on where their child might go and whom they might see.”

As an example, he said this could include the parents of a 14-year-old girl who wanted to go out “publicly dressed in what might be regarded, by her parents as an overly sexualised and sexually provocative and explicit way.”

The proposed legislation, Mr O’Neill argued would mean their “parental action is stopping the child from living or acting in accordance with how their child wishes to express their (hetero)sexual orientation and/or (cis)gender identity."

READ MORE: Conversion therapy: Minister accuses SNP MSP of supporting torture

On religious freedom, Mr O’Neill said the proposals would have “the undoubted effect of criminalising much mainstream pastoral work of churches, mosques and synagogues and temples.”

He also said any medical professional who tried to “dissuade an individual against undergoing or undertaking medical procedures (such as puberty blockers, hormone treatment and/or surgeries) which are associated with, and intended to further, gender reassignment,” could also end up in legal difficulties.

Feminist groups who are critical of gender identity politics could also be prohibited from speaking out “on any matters relating to sexuality and/or gender, which conflicted with any of the official positions now adopted by the State”.

Mr O’Neill said the proposals were “ill-thought out, confused and confusing, and fundamentally illiberal in intent and effect.”

He said it was "best described as ‘jellyfish legislation’."

As the "concepts it uses are impossible to grasp; its limits are wholly undefined; it contains a sting in the tail in the form of criminal sanction of up to seven years and unlimited fines; and thus it will have an undoubted and intended effect of dissuading persons from ever even entering the now murky waters of what may or may not constitute unlawful ‘conversion practices’.”

He added: “I conclude therefore that there are very strong arguments indeed that these legislative proposals of the Scottish Government are beyond the legislative competence of the Scottish Parliament, primarily because of their over-breadth, their disproportionate intrusion into private and family life and freedom of religion and freedom of expression, but also because of their internal incoherence.”

Simon Calvert from the Christian Institute said: “This is another example of the Scottish Government asking Holyrood to exceed its powers and impose draconian legislation on the people.

“If this deeply flawed law is passed it will be challenged all the way to the Supreme Court if necessary.

“Ordinary mums and dads face criminalisation for trying to extract their kids from the grip of radical trans ideology. Church leaders face prosecution for not praying in accordance with state dogmas. This is outrageous.

“They’ve spent over two years drafting this. It’s time to admit that no-one can define what conversion therapy is, let alone how to criminalise it.”

READ MORE: Proposals to ban conversion therapy published by Scottish Government

A Scottish Government spokesperson said: “As set out in the consultation, our proposals do not prohibit parental and religious guidance that is not motivated by an intention to change or supress their sexual orientation or gender identity.

“Our approach fully respects the existing legal right to a private and family life, and the freedoms of speech, religion and belief.

“This is a consultation, and the Scottish Government is open to all views.”

The ban is a key plank of the Bute House Agreement between the SNP and the Scottish Greens.

Last month, Blair Anderson a survivor of conversion therapy and Green councillor, told The Herald that a ban on conversion therapy "is Scotland’s chance to say clearly to LGBTQ+ people: you are worthy of safety, love and respect exactly as you are."