"There's going to be a lot of drama," said a senior C of E source. "It's 90% likely that the six will walk out. If we get past Tuesday, we'll be doing well."

The meeting of Anglican primates was called by Justin Welby in a last-ditch effort to move the global church -- which claims 85 million followers -- beyond the issue of homosexuality in order to focus on other pressing matters such as religious violence and climate change. Welby is proposing that, in the face of intractable differences, the communion reshapes itself as a loose confederation of churches rather than adherents to a common doctrine.

But the six African churches are insisting on sanctions against the US Episcopal Church, which tipped the simmering conflict over gay rights into open hostility when it consecrated gay priest Gene Robinson as bishop of New Hampshire in 2003.

This week, Archbishop Stanley Ntagali -- leader of the Anglican church in Uganda, which has backed the criminalisation of homosexuality in the east African country -- warned that he would walk out of the primates' meeting if "discipline and godly order is not restored".

Archbishop Eliud Wabukala of Kenya said the "truth [of the Gospel] continues to be called into question in the Anglican communion" and warned against "the global ambitions of a secular culture".

Peter Jensen, general secretary of Gafcon, a group of conservative Anglican churches formed to "guard and proclaim the unchanging Gospel", said in a new year message: "Truth matters even more than institutional unity."

The first potential trigger for a walkout will be when the order of the agenda is decided at the start of the summit. Conservative primates are insisting that the issue of sexuality is discussed first; in the unlikely event that they do not prevail, they may leave in protest. The C of E considers this a "soft walkout".

More significant would be a "hard walkout" on the issue of disciplining the US Episcopal Church. This could lead to a formal rather than de facto schism, with conservative churches around the world realigning under the authority of Gafcon. If the six leaders wish to formally detach their provinces from the Anglican communion, each needs to embark on a lengthy process authorised by their churches.

The C of E believes there is an 80% chance of a "hard walkout" and puts the likelihood of a walkout of either kind at 90%. The primates' meeting will continue throughout the week, regardless of departures.

Welby is said to be phlegmatic about the prospect, believing he has done everything possible to offer the opportunity to forge a new, looser relationship, which hardliners may choose to reject. "His mood is not 'Crisis, what crisis?' but 'Crisis? Well, what's new?'," said the source.

C of E officials have also averted a threatened boycott of next week's meeting by the more liberal wing of the Anglican communion, following a controversial invitation from Welby to the leader of the conservative breakaway Anglican Church in North America (ACNA) to participate in the meeting.

Despite having no formal remit, ACNA archbishop Foley Beach will be permitted to lobby for his staunch opposition to same-sex marriage at the Canterbury gathering. However, he will be asked to withdraw when decisions are taken.

C of E leaders acknowledge that the issue of homosexuality has fractured the communion, but believe that a looser relationship of churches linked to Canterbury yet not to each other is the only way to overcome institutional dysfunctionality.

Only eight of the 38 provinces are open to changing doctrine on marriage to allow for same-sex unions, leaving a large majority in favour of keeping marriage exclusively as a union between one man and one woman. Those eight are the US, Canada, Scotland, Wales, New Zealand, South India, South Africa and Brazil.

Welby, who visited all 38 Anglican provinces within the communion in the two years after his enthronement, telephoned each leader from his French holiday home last summer to persuade them to take part in the primates' meeting. Although such summits are supposed to be held every two years, the last was in 2011.

All the leaders have accepted Welby's invitation, but two or three are expected to miss the meeting owing to ill-health. The gathering will combine prayer with discussion, although different groupings will have the opportunity to pray in separate areas if they wish.

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