LGBT protesters are holding a vigil outside the Church of England's parliament on Wednesday morning ahead of a debate on gay relationships.
The CofE's governing body, the General Synod, will discuss a report by bishops refusing to change their view that marriage is between one man and one woman.
But the Church's hierarchy are deeply concerned a tide of anger against the report will lead to it being thrown out in a vote later on Wednesday.
Officials have been ringing bishops to ask how their members – the equivalent of MPs – intend to vote.
Top bishops have also been called on to blog regularly on the importance of passing the report.
But campaigners say they are 'quietly confident' the report will be voted down.
Peter Tatchell, who is organising the protest alongside Anglican pressure groups Changing Attitude and the Lesbian and Gay Christian Movement, said he was 'appalled' by the Church's treatment of LGBT people.
'The church blesses dogs and cats but it refuses to bless loving, committed same-sex couples,' he said. 'It treats LGBTI clergy and laity as second class, both within the Church and the wider society.'
Tracey Bryne, chief executive of the LGCM, said: 'We need the bishops to face up to what a "change of tone" would mean: Not more words but a recognition of the real damage that continues to be done by this inertia.
'We need a national programme of practical change to move the Church forward. That will require resources – a national lead person – and LGBT people at every level of the strategy, holding the church to account and monitoring effectiveness on the ground.'