Scotland's most senior Catholic says that if the government goes ahead allows gay and lesbian couples to adopt, their children are likely to become homosexual.

Writing in Scotland's Sunday Times, Cardinal Keith O’Brien cites research that most international psychiatric groups have discredited that purports to back up his claim.

One of the "authorities" O'Brien quotes is Aquilino Polaino, a psychology professor at Madrid's Catholic university, who earlier this year told the Spanish Senate that children of gays suffer a significant increase in low self-esteem, stress, confusion over sexual identity, increased mental illness, drug use, promiscuity, and sexually transmitted infections.

Polaino had been invited by the conservative opposition party to address a Senate committee that was studying same-sex marriage. The opposition later apologized for Polaino's remarks but the Senate voted to reject the bill, forcing the government to submit it to an override vote in the lower house.

Several American studies have shown that the children of gays are as well adjusted as those of heterosexuals.

Nevertheless, O'Brien, in his Sunday Times remarks said: "We ignore a wealth of global evidence and place innumerable children in peril if we forget certain immutable truths; children need a male and a female role model in a permanent relationship.

"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."

O'Brien also said that sex relationships were “unstable”.

The Scottish executive has announced plans change the existing law that permits only married couples to jointly adopt.

British LGBT group Stonewall accused the cardinal being out of touch.

"We are saddened that a senior cleric is exhuming 19th Century prejudices rather than recognizing the reality of life in the 21st Century," said Ben Summerskill, Stonewall chief executive.

"There's no credible evidence to support the cardinal's view that the educational and social development of the children of gay and lesbian couples is any different from that of the mainstream population."