The head of the Roman Catholic Church in Scotland has hit out at the Government’s plans to redefine marriage.

The Government is launching a consultation this month on changing the definition of marriage to apply also to homosexual unions.

There has been a strong backlash against the proposals among the public and church leaders.

Writing in the Sunday Telegraph, Cardinal Keith O’Brien said the impact of changing the definition of marriage would be “immense”.

“Since all the legal rights of marriage are already available to homosexual couples, it is clear that this proposal is not about rights, but rather is an attempt to redefine marriage for the whole of society at the behest of a small minority of activists,” he said.

“Redefining marriage will have huge implications for what is taught in our schools, and for wider society. It will redefine society since the institution of marriage is one of the fundamental building blocks of society. The repercussions of enacting same-sex marriage into law will be immense.”

The cleric warned that if the law were to be changed, those who openly oppose the new definition could find themselves the “next victims of the tyranny of tolerance” and branded “heretics” in the face of “state-imposed orthodoxy”.

He described the proposals as "madness" and a “grotesque subversion of a universally accepted human right”, as he warned that anyone who upholds a traditional understanding of marriage risks being labelled an “intolerant bigot”.

“As an institution, marriage long predates the existence of any state or government.

“It was not created by governments and should not be changed by them.

“Instead, recognising the innumerable benefits which marriage brings to society, they should act to protect and uphold marriage, not attack or dismantle it.”

Cardinal O’Brien criticised the way in which the viewpoint of the child had been “completely lost” in the debate on gay marriage.

“Same-sex marriage would eliminate entirely in law the basic idea of a mother and a father for every child,” he said.

“It would create a society which deliberately chooses to deprive a child of either a mother or a father.”

He warned that the legalisation of same-sex marriage could lead to the legalisation of polygamy further down the road.

“If marriage is simply about adults who love each other, on what basis can three adults who love each other be prevented from marrying?”

He went on to say that the Government’s promise that same-sex marriage would not be compulsory for churches was disingenuous and “staggeringly arrogant”.

“Imagine for a moment that the Government had decided to legalise slavery but assured us that ‘no one will be forced to keep a slave’," he said.

“Would such worthless assurances calm our fury? Would they justify dismantling a fundamental human right? Or would they simply amount to weasel words masking a great wrong?

“The Universal Declaration on Human Rights is crystal clear: marriage is a right which applies to men and women, “the family is the natural and fundamental group unit of society and is entitled to protection by society and the State”.

“This universal truth is so self-evident that it shouldn’t need to be repeated. If the Government attempts to demolish a universally recognised human right, they will have forfeited the trust which society has placed in them and their intolerance will shame the United Kingdom in the eyes of the world.”