The head of the Evangelical Alliance UK has appealed to the Church of England to reaffirm its historic commitment to heterosexual marriage.

In an open letter published in Christianity magazine, Gavin Calver said that he was "saddened, pained and a little shocked" by the proposals unveiled last week and that the proposals are an "unsustainable way forward".

"After a long process of listening and discussion, proposals are being brought to General Synod that, while claiming not to change their doctrine of marriage, do appear to dramatically compromise the church's teaching and practice on relationships and sexual ethics," he wrote.

The Church of England said last week that same-sex couples will not be allowed to marry in CofE churches but will be able to have a service of blessing.

The Prayers in Love and Faith unveiled by the CofE last week include prayers for thanksgiving, dedication and God's guidance.

They will be debated by the Church of England's parliamentary body, the General Synod, next month. 

The open letter has been published after meetings between Calver, the Church of England Evangelical Council and Lambeth Palace, the official residence of the Archbishop of Canterbury.

In his letter, Calver further warned that the ramifications of introducing blessings for same-sex couples will be felt far beyond the Church of England.

He is asking evangelicals to pray ahead of next month's Synod debate. 

"As we have listened, it has been abundantly clear that there is a wide range of views and opinions held and that navigating a way forward is fraught with challenges," he wrote.

"This is not an easy conversation, there is real pain and hurt to be acknowledged and repented of. But, it is also profoundly clear that the attempted compromise being suggested is entirely inadequate for all sides of the debate.

"The proposals being suggested are an unsustainable way forward."