The head of the Roman Catholic Church in Scotland fears that schools will be banned from promoting traditional marriage if gay marriage is legalised.

Cardinal Keith O’Brien was responding to a legal opinion from European human rights and employment law specialist, QC Aidan O’Neill.

In his opinion, Mr O’Neill said parents will find it difficult to prevent their children from attending classes that promote homosexual marriage.

“The extension of marriage to same sex couples is not one currently required under and in terms of the European Convention of Human Rights,” he said.

“But if this is done, the case law of the European Court of Human Rights makes it clear that parents with older traditional and often religiously based views which insist on no marriage without committed and faithful heterosexual sex – will be hard-pressed to rely upon their Convention rights as parents to ensure education and teaching by the state of their children in conformity with their own religious and philosophical convictions.”

The legal opinion was sought by Scotland For Marriage, the advocacy group spearheading opposition to the proposed change in law north of the border.

The group fears that classrooms will be flooded with teaching materials, including books and pamphlets, that promote gay marriage.

Mr O’Neill said: “The way in which the curriculum is delivered by school is covered by the anti-discrimination provisions of the Equality Act 2010 such that pupils should not be taught in a manner which may be thought to subject them to discrimination on the basis of sexual orientation.

“Scottish Ministers and local education authorities have obligation to have “due regard” to the need to tackle prejudice and promote understanding in relation to matters of different sexual orientation.

“This duty applies regardless of whether marriage be opened to same sex couples.”

Cardinal O’Brien said a change in the law would “stigmatise” the beliefs of Christians and other people of faith, and “open the door to persecution and discrimination”.

"It is clear that Scotland's schools will be banned from promoting a traditional understanding of marriage if same sex marriage becomes law," he said.

"This means that the reassurances offered by the Scottish Government that schools and churches will be protected have been utterly hollow.

"I would urge the Scottish Government not to make promises they cannot fulfil or offer protection it is not in their power to provide.

“If they enact Same Sex Marriage legislation there is no question that the work of Catholic schools and parishes will be impaired, undermined and damaged."

A Scotland For Marriage spokesman said: "This is a damning verdict which demonstrates the far-reaching and divisive impact of the proposed legislation. It is paramount that the rights of ordinary parents are not ignored.

"In addition, we believe there will be further ramifications. There will be serious questions about our teachers. If the legislation is approved and marriage is redefined we hold genuine fears that teachers may find themselves open to disciplinary action if they voice their support for traditional marriage in the classroom."