The Bishop of Salisbury, the Right Reverend Nicholas Holtam, has met with criticism after reiterating his support for gay marriage in a strongly worded letter published in The Daily Telegraph.

His intervention comes in spite of the Church of England's official opposition to legalising gay marriage.

The Marriage (Same Sex Couples) Bill will be debated next week in the House of Lords.  The bishop made headlines last year when he spoke publicly about his support for gay marriage.

In a letter to Lord Alli of Norbury, published in The Daily Telegraph today, Bishop Holtam said legalising gay marriage was a "very strong endorsement" of the institution of marriage.

"The possibility of 'gay marriage' does not detract from heterosexual marriage unless we think that homosexuality is a choice rather than the given identity of a minority of people," he wrote.

"Christian morality comes from the mix of Bible, Christian tradition and our reasoned experience. Sometimes Christians have had to rethink the priorities of the Gospel in the light of experience.

"For example, before Wilberforce, Christians saw slavery as Biblical and part of the God-given ordering of creation.

"Similarly in South Africa the Dutch Reformed Church supported Apartheid because it was biblical and part of the God-given order of creation. No one now supports either slavery or Apartheid. The biblical texts have not changed. Our interpretation has."

His comments have been criticised by Vinay Samuel, founder of the Oxford Centre for Missions Studies, and Chris Sugden, executive secretary of Anglican Mainstream.

In a joint statement, they said it was "unfortunate" that Bishop Holtam was "setting aside the clear teaching of the Bible, acknowledged by the whole church, to accommodate the church's situation in a culture that has become sexually morally lax".

They challenged his view of homosexuality as an innate characteristic, saying there was no evidence that this was the case.

They point to the findings of Dr Neil Whitehead, former researcher for the New Zealand government, who looked at eight major studies of identical twins in Australia, the US, and Scandinavia. He found that the chances of a gay twin's co-twin also having same-sex attractions was only around 11% for men and 14% for women.

Samuel and Sugden continued: "To base an argument and change an established law that has enabled families and communities to flourish for generations on such ill-founded evidence is not acceptable especially for a church leader."

Earlier this week, the evangelical group Church Society called on the House of Lords to reject the Marriage (Same Sex Couples) Bill.

Lee Gatiss, Director of Church Society, said the Bill was "ill-conceived" and would "undermine marriage" as well as lead to "unforeseen" consequences for decades to come.

"The Bill does not merely 'open up' marriage to a new group of people, as many are portraying it," he said.

"Rather, it enforces and compels everyone in our country to hastily accept the creation of an entirely new socio-legal structure for all of us, which possesses neither an underlying consensual basis or a democratic mandate, and is actually only sought by a small minority of the gay community."

The Bishop of Salisbury's comments are in defiance of the Church of England's official position calling for the traditional definition of marriage to remain unchanged. The Church told MPs in a briefing in February that it could not support gay marriage because of the "uncertain and unforeseen consequences for wider society and the common good".

The Church said the Government's plans were an "unnecessary politicising" of marriage and that it had "no mandate" to change the law.