A briefing has been sent to MPs and peers explaining why the definition of marriage should not be changed.

It has been drawn up by Catholic bishops who are urging members of Parliament to oppose the Marriage (Same Sex Couples) Bill when it has its second reading.

They warn that the bill "fundamentally seeks to break the existing legal link between the institution of marriage and sexual exclusivity, loyalty and responsibility for the children of the marriage".

"But for children, there would be no need of any institution concerned with sex," the bishops say.

"It is through children alone that sexual relations become of importance to society, and worthy to be taken cognizance of by a legal institution."

The bishops further state that a traditional understanding of marriage is not discriminatory.

"Those who argue for same sex marriage do so on the basis that it is unjust to treat same sex and heterosexual relationships differently in allowing only heterosexual couples access to marriage.

"Our principal argument against this is that it is not unequal or unfair to treat those in different circumstances differently. Indeed, to treat them the same would itself be unjust."

The briefing goes on to say that the Government has no mandate to make such a major constitutional change and Parliament should not be rushed into changing the law.

The bishops also raise concerns about inadequate safeguards for those with a traditional understanding of marriage and the impact of changing the law on freedom of expression and religion, particularly in education.

"Parliament may seek to provide protections for religious individuals or religious organisations under domestic law but it cannot ensure that these protections themselves will withstand complaints against them to the European Court of Human Rights," they say.

They add: "The consequences of the bill will be wide-ranging. The Government has not identified all these consequences and they certainly have not all been addressed."