The Scottish Episcopal Church will today decide whether to legalise gay marriage in church and become the first British Anglican Church to do so.

The landmark vote at its governing general synod in Edinburgh is the final stage in changing the teaching on marriage as solely between 'one man and one woman'.

The motion, which is expected to pass, is likely to have serious consequences for its relationships with the global Anglican Communion.

A group of conservative Anglicans, deeply opposed to the change, will announce a 'missionary bishop' minutes after the vote is announced, effectively setting up a rival Anglican Church in the UK.

The move by GAFCON means traditionalist parishes can pledge allegiance to the new bishop, rather than their official local bishop.

Justifying the move, GAFCON released a statement saying: 'If this action is taken by the SEC it will further marginalise faithful Anglicans in Scotland who seek to uphold Jesus' teaching on marriage.'

Scottish church leaders have sought to stave off a split by allowing a 'conscience clause' meaning individual priests are not obliged to carry out same-sex weddings if they don't want to.

Speaking ahead of the General Synod, Most Rev David Chillingworth, Primus of the SEC, said: 'The Scottish Episcopal Church, in common with all other churches, expresses a diversity of views on this question.

'Those views are held with integrity. Our church will seek to reach a decision on the canonical question while sustaining its unity in Christ.

'To do so will require both humility and generosity on all sides.'

The change in cChurch law comes after the SEC's synod last year agreed to send the matter to the seven local diocesan synods for discussion.

The issue now comes back to the main general synod with six out of seven having voted in favour of amending the law.

If passed the SEC would remove the understanding of marriage as 'a physical, spiritual and mystical union of one man and one woman'.

Rev David McArthy, a traditionalist priest in the SEC and part of the conservative Scottish Anglican Network, told Christian Today there was an 'immense sadness from many people in Scotland' about the upcoming decision.

'It is not simply a group of evangelical churches who have concerns about this but a fairly wide group,' he said.

'I pray that the leadership realise what they are about to do will have serious consequences for the Church.'

But supporters of gay marriage hope to see same-sex couples married in church by the end of year and see it as Scotland leading the way in the Church's debate.

When threatened with 'consequences' by the Archbishop of Canterbury if they went ahead, Chillingworth responded by saying we 'will not change what we do'.

He added: 'Maybe it is a price worth paying for the ultimate healing of the [Anglican] Communion.'