The leader of the Catholic Church in England and Wales has expressed “astonishment” at claims that new minister for Women and Equality will struggle to fulfil her role because of her faith.

In a letter to The Times, Cardinal Cormac Murphy O’Connor, the Archbishop of Westminster, asked for respect for Ms Kelly’s beliefs.

He wrote, “Sir, I am astonished that Ruth Kelly’s Catholicism might be considered incompatible with her role as Equality Minister.

“The Catholic Church teaches that some actions are sinful, sexual acts outside marriage among them. St Thomas Aquinas taught that not every sin is necessarily a crime, and not every crime is necessarily a sin. From this stems the Church’s defence of human rights. Homosexual people are first of all persons, and have the same entitlement to legal rights as anyone else.

“The Church has consistently spoken out against any discrimination against homosexual persons, and will continue to do so. Every politician needs to balance the demands of his or her conscience with the need for collective responsibility in Government. Ruth Kelly is finding that balance for herself. For this she deserves respect, not criticism. Ms Kelly may well be scrutinised for her fitness for office. That is a political judgment; her Catholicism should not be a criterion in that judgment.”

Ms Kelly, 39, has faced national criticism this week for her poor voting record regarding gay rights and membership of Opus Dei, a Christian group which views homosexuality as a sin.