Although the vote is non-binding, the upcoming result's been billed as either the beginning of the end to discussions to changes on Anglican doctrine, or the start of more concerted efforts to revise it.
Rev Peter Ould, a commentator on issues of sexuality, told Premier beforehand: "It may turn out that Wednesday's vote is the beginning of that drawing a line in the sand and saying: 'okay, this is what we believe now - you either get with the programme and help work with us within that framework, or you move on'."
A report by the House of Bishops, which will be presented to Synod members on Wednesday, calls for the Church to adopt a "fresh tone and culture of welcome and support" for gay people, but not to change its opposition to same-sex marriage.
The report will be the subject of a "take note" debate, which means Synod will discuss the content and recommendations of the report to inform future work, but the proposals will not be formally rejected or approved.
Published last month, the report prompted 14 retired bishops to come out in opposition, saying the "authentic voices" of LGBT people were being ignored.
Former Bishop of Worcester, Rt Revd Peter Selby, told Premier ahead of today's vote: "My wife said several years ago when she was teaching vulnerable young people - and the Church came out with one of these things - she said: 'what that does is tell them that really we have nothing to offer'.
"We have a lot to offer, a gospel, an attractive and loving person who values their presence and their being."
Demonstrators calling for a change in church tone and doctrine on marriage and sexuality have been protesting outside Church Hall, Westminster:
Campaigner Peter Tatchell said: "The Bishops' report defends heterosexual superiority and opposes same-sex blessings and marriages.
"The church blesses dogs and cats but it refuses to bless loving, committed same-sex couples.
"It treats LGBTI clergy and laity as second class, both within the church and the wider society."
Peter Saunders, head of the Christian Medical Fellowship and a theological commentator, said: "Within the Church, it is now very clear that the two main positions held are irreconcilable.
"The problem now is that the church, and especially its ruling body the General Synod, is now so full of people practising, promoting and defending this particular form of sexual immorality that nothing short of radical surgery will resolve it.
"The time for shared conversations is over. It is now time to act."
