THE Catholic Church yesterday hit back at the Scottish Government over plans to allow same-sex marriage, claiming "no clear-headed person" would condone the concept and raising potential grounds for a legal challenge.

Archbishop of Glasgow Mario Conti, in his first response to the SNP administration's confirmation of the law change to allow same-sex marriages, said the Holyrood legislation is flawed and not competent.

In a letter to The Herald, he writes: "The Scottish Government's decision to introduce 'gay marriage' is mistaken for the reason that it fails ... to distinguish between equality and sameness.

"No clear-headed person is going to say that a partnership of two men, or of two women for that matter, is the same reality in all essential respects as the conjugal relationship of a man and a woman."

The senior churchman spoke out days after his successor, Archbishop-elect Philip Tartaglia, was criticised over remarks made about the late Labour MP David Cairns, who was gay.

In his response, Archbishop Conti said: "No government has the competence to declare that two realities which differ significantly in their moral and social character are the same and indeed there are no imperative grounds in any declaration of human rights to countenance it, thus subverting the universally accepted definition of marriage."

He said the plan fails "to distinguish between equality and sameness" and added it would split Scotland.

"It will not be a united country when a significant number of its citizens, whether a majority or a minority, on grounds both of reason and of faith, are opposed and will continue to be opposed to this unwise and unnecessary action of the Scottish Government," said the Archbishop.

"I believe more respect would be accorded across society to such partnerships when their integrity is expressed in a way which acknowledges their difference.

"I thought it was also the Scottish Government's concern that both equality and diversity should be respected in Scottish society."

It came as Green MSP Patrick Harvie criticised the head of the Scottish Catholic media office for comments he made on Scotland Tonight on Wednesday.

In an open letter to Peter Kearney the MSP said Mr Kearney stated on TV there exists a "vast array of medical evidence - to suggest that same-sex behaviour is hazardous, is harmful, and is dangerous".

Mr Harvie continued: "I hope that on reflection you can see the problem with this statement," adding: "Whether this line of argument has any bearing on the same sex marriage debate is unclear."

Speaking about Mr Kearney's comments, Stonewall Scotland Director Colin Macfarlane said: "It's a great shame that when the Church could be campaigning on issues that really concern Christians – like global poverty, hunger and conflict – one of its leading communicators instead spends much of his time gratuitously insulting same-sex couples and spreading malicious smears about gay people's health."

Mr Kearney said he would respond "in due course".

Earlier this week Archbishop-elect Tartaglia was accused of having made unacceptable and homophobic remarks about the deceased MP Mr Cairns.

Mr Cairns's partner Dermot Kehoe called on Bishop Tartaglia, the current Bishop of Paisley, to apologise after comments he made linking the Labour politician's death to his homosexuality. Bishop Tartaglia made the comments earlier this year.