Gay partnerships pose the same ethical questions as those between men and women, and the key issue for Christians is that they are faithful and lifelong, he believes.
Dr Williams is known to be personally liberal on the issue but the strength of his views, revealed in private correspondence shown to The Times, will astonish his critics.
The news threatens to reopen bitter divisions over ordaining gay priests, which pushed the Anglican Communion towards a split.
As Archbishop of Canterbury, Dr Williams recommitted the Anglican Communion to its orthodox position that homosexual practice is incompatible with Scripture at the Lambeth Conference, which closed on Sunday.
However, in an exchange of letters with an evangelical Christian, written eight years ago when he was Archbishop of Wales, he described his belief that biblical passages criticising homosexual sex were not aimed at people who were gay by nature.
He argued that scriptural prohibitions were addressed to heterosexuals looking for sexual variety. He wrote: “I concluded that an active sexual relationship between two people of the same sex might therefore reflect the love of God in a way comparable to marriage, if and only if it had about it the same character of absolute covenanted faithfulness.”
Dr Williams described his view as his “definitive conclusion” reached after 20 years of study and prayer. He drew a distinction between his own beliefs as a theologian and his position as a church leader, for which he had to take account of the traditionalist view.
The letters, written in the autumn of 2000 and 2001, were exchanged with Deborah Pitt, a psychiatrist and evangelical Christian living in his former archdiocese in South Wales, who had written challenging him on the issue.
In reply, he described how his view began to change from that of opposing gay relationships in 1980. His mind became “unsettled” by contact as a university teacher with Christian students who believed that the Bible forbade promiscuity rather than gay sex.
Dr Williams, who was ordained a priest in 1978, became a lecturer at Cambridge two years later and was appointed Dean of Clare College in 1984.
He told Dr Pitt that by the end of the 1980s he had “definitely come to the conclusion” that the Bible did not denounce faithful relationships between people who happened to be gay.
He cited two academics as pivotal in influencing his view. One of them was Jeffrey John, the celibate homosexual whom he later forced not to become Bishop of Reading after an outcry from conservatives.
In his 1989 essay The Body’s Grace, Dr Williams argued that the Church’s acceptance of contraception meant that it acknowledged the validity of nonprocreative sex. This could be taken as a green light for gay sex.
Liberals have been bitterly disappointed that a man whom they regarded as chosen to advance their agenda has instead abided by the traditionalist consensus of the majority.
In the correspondence Dr Williams wrote of his regret that the issue had become “very much politicised” and was treated by many as “the sole or primary marker of Christian orthodoxy”.
Asked to comment yesterday, Lambeth Palace quoted a recent interview in which the Archbishop said: “When I teach as a bishop I teach what the Church teaches. In controverted areas it is my responsibility to teach what the Church has said and why.”
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