The Bishop of Salisbury has likened opponents of equal marriage to Christians who used the Bible to justify slavery and apartheid.

The Rt Rev Nicholas Holtam previously opposed marriage rights for same-sex couples, but is now the only Anglican diocesan bishop in the country to publicly favour the reform.

Last year he said he was “no longer convinced” marriage should be between a man and a woman.

Peers will begin debating the Marriage (Same Sex Couples) Bill as part of its second reading in the House of Lords on Monday.

In a letter sent to Lord Alli, a gay Labour Muslim peer, which has been published by the Telegraph, Bishop Holtam distanced himself from the Church of England’s official opposition to marriage equality.

He said: “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.”

Lord Alli asked the bishop to set out his views for the benefit of peers debating the bill.

Bishop Holtam said: “You, as a gay Muslim, will not be surprised that there are a variety of views within the Church of England where we are experiencing rapid change similar to that in the wider society.”

He added: “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.

“Indeed the development of marriage for same-sex couples is a very strong endorsement of the institution of marriage.”

Bishop Holtam added that the so-called “quadruple locks” – prescribed by the government to protect religious groups, including the Church of England, who do not wish to carry out same-sex ceremonies, are “extraordinarily robust”.

The Church of England, which has 26 bishops in the Lords, formally opposes equal marriage and there has been speculation that the Archbishop of Canterbury Justin Welby will be among bishops voicing their concerns about the policy during next week’s debate.