The Archbishop of Canterbury, Dr Rowan Williams, has asked Anglicans worldwide to pledge to maintain unity over gay bishops and same-sex marriage.

The Queen is due to open the General Synod of the Church of England at Westminster Abbey today for the 476 members to debate David Cameron’s vision of the “Big Society”.

One of the items for debate is the Anglican Covenant, an agreement first discussed in 2004 after the election of the openly gay Bishop of New Hampshire, Gene Robinson.

The covenant is an attempt to maintain unity between conservative and liberal factions of the church, who have clashed over acceptance of homosexuality.

It asks member churches not to act in any way which will upset other churches. This could involve not ordaining gay bishops or blessing same-sex marriages or civil partnerships.

Liberal church groups Modern Church and Inclusive Church are opposed and say it could be “the biggest change to the church since the Reformation”.

In an advert in Church Times last month, they argued that the Church of England should not be subject to “outside power” and that signing the agreement will make churches “less inclusive” and “more backward-looking”.

In response, the Bishop of St Asaph, the Rt Rev Gregory Cameron, who was on the committee that drew up the covenant, wrote to the Church Times to accuse them of being “latter-day little Englanders” and the “nearest to an ecclesiastical BNP that I have encountered”.

Dr Williams told the General Synod: “It is an illusion to think that without some changes the Communion will carry on as usual. The unpalatable fact is that certain decisions in any province affect all.”

He added: “If we ignore this, we ignore what is already a real danger, the piece-by-piece dissolution of the Communion and the emergence of new structures in which relation to the Church of England and the See of Canterbury are likely not to figure significantly.”

Rev Robinson announced earlier this month that he will step down in 2013, seven years earlier than expected.

In an interview last week, he said it appeared that Dr Williams had been “abducted by aliens” over the change in his views towards homosexuality.

He said: “We were dancing in the streets when Rowan Williams was made Archbishop of Canterbury. We just thought it was a wonderful choice.

“And we are so perplexed here in the American Church about what he has done, what he has said.

“I have clergy friends in England who literally studied at Archbishop Williams’s feet when he was teaching and who have said to me it is almost as if aliens have come and taken Rowan away from us and they have left something here that looks like him but we don’t recognise him any more.”