The leader of the Anglican Church in Nigeria has slammed the General Synod of the Church of England for 'false teaching' and is warning that it is in 'grave spiritual danger'.

Archbishop Nicholas Okoh's attack follows the Synod's decision to back a proposal for services marking new identities for transgender people.

Writing to the 'faithful' of the conservative GAFCON fellowship that he chairs, Okoh says in his latest letter, published on the website the Anglican Church in NIgeria, 'False teaching is restless and relentless, and the Church of England itself is in grave spiritual danger.'

He says it 'much to be regretted' that there has been far more concern about alleged 'boundary crossing' than about the 'contempt of God's Word' that made the controversial consecration of missionary bishop Andy Lines necessary.

Condemning also the 'recent capitulation of the Scottish Episcopal Church to secular ideas about marriage'. he criticises the fact that the Bishop of Edinburgh, who has strongly supported the Scottish Episcopal Church's move towards endorsing same sex marriage, was invited as a guest of honour the synod which met a few days ago in York.

'Although the Church of England's legal position on marriage has not changed, its understanding of sexual morality has,' he writes. 'Same sex relationships, which were described by Lambeth Resolution I.10 of 1998 as "incompatible with Scripture" now receive approval at the highest level.

'For example, Vicky Beeching, a singer, songwriter and activist who advocates homosexual marriage was honoured with the Archbishop Thomas Cranmer award for Worship in a ceremony at Lambeth Palace.'

He says the need for GAFCON to safeguard the integrity and clarity of global Anglican mission is as urgent as it has ever been, he writes. 'Our calling is not to be conformed, but to be transformed. A watching world needs to know that Anglicans are defined first and foremost by faithfulness to the Word of God.'

Jayne Ozanne, a member of the General Synod who introduced the motion calling for a ban on gay conversion therapy, said: 'It is with great sadness that I have read Archbishop Okoh's inflammatory words regarding the thoughtful decisions made by the General Synod, endorsed by both our Archbishops, in relation to the LGBTI community. He does himself no credit in calling those with whom he disagrees "false teachers", and would do well to reflect on how broad and deep the love of God is for all humankind.

'GAFCON continues to alienate itself from those open to hearing what the Spirit is saying, and who look to take the whole Bible into account – not just a few chosen verses.'