Cardinal Keith O’Brien said the recommendations of a Scottish executive expert group threatened to place children in an unstable environment that was “gravely immoral”.

He cited research which, he claims, shows children adopted by same-sex couples exhibit “behavioural change”, including “low self-esteem, stress, confusion regarding sexual identity, increased mental illness, drug use, promiscuity, sexually transmitted infections and homosexual behaviour”.

Writing in today’s Sunday Times Scotland, O’Brien warns: “Scotland’s adopted children must not become guinea pigs in some distorted social experiment aimed at redefining marriage, subverting the family and threatening the good of society.”

His comments are the most outspoken attack on homosexuality since his predecessor, Cardinal Thomas Winning, described gay sex as “perverted”. Winning was instrumental in the deeply divisive campaign to repeal section 28, which banned the promotion of homosexuality in schools.

The Scottish executive plans to legislate to change the law, extending rights of adoption. At present only married couples are allowed to jointly adopt. Unmarried and same-sex couples can be assessed for adoption but only one partner can legally adopt. A change in the law would bring Scotland in line with England which introduced similar changes two years ago.

O’Brien said same sex relationships were “unstable” and warned that ministers were disregarding the needs of children.

“We ignore a wealth of global evidence and place innumerable children in peril if we forget certain immutable human truths; children need a male and a female role model in a permanent relationship,” he said.

Ben Summerskill of Stonewall, a gay pressure group, said there was “no credible evidence” to support O’Brien.

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