LONDON (Reuters) — One of Africa's most outspoken archbishops issued a veiled threat on Wednesday to boycott a major Anglican conference over the deeply divisive issue of gay clergy.American liberals, who sparked the row in the first place by ordaining an openly gay bishop, called for calm among the world's 77 million Anglicans — but the plea fell on deaf ears.

The latest crisis on the road to schism was provoked by Archbishop of Canterbury Rowan Williams, the church's spiritual leader who has battled to placate the warring camps and bemoaned what many see as an Anglican obsession with sex.

On Tuesday, Williams sent invitations to more than 800 Anglican bishops asking them to attend the Lambeth Conference in July and August, 2008 in Canterbury in England.

But he did not invite two American bishops, Gene Robinson and Martyn Minns, to the once-in-a-decade summit.

The openly gay Robinson has caused division since he was consecrated as bishop of New Hampshire in 2003.

Minns, a deeply conservative Episcopalian, was installed last year as the head of a new Nigerian-based church branch in the United States designed as a refuge for orthodox believers. The Anglican Communion does not recognise his position.

Williams' decision provoked a fiery response from Nigeria's Archbishop Peter Akinola.

"The withholding of invitation to a Nigerian bishop, elected and consecrated by other Nigerian bishops, will be viewed as withholding invitation to the entire House of Bishops of the Church of Nigeria."

US Bishop Katherine Jefferts Schori, leader of the liberals, called for calm and said: "It is possible that aspects of this matter may change in the next 14 months."

But, judging by the furious reaction to Williams' decision, the chances of bridging the yawning chasm between liberals and conservatives look as slim as ever.

In the liberal camp, Integrity USA President Susan Russell accused Williams of lacking leadership, by not inviting Robinson, and said: "He has allowed himself to be blackmailed by forces promoting bigotry and exclusion in the Anglican Communion."

Minns, in turn, said: "It should be remembered that this crisis in the Anglican Communion is not about a few individual bishops but about a worldwide Communion that is torn at its deepest level."

British religious commentator Clifford Longley, reflecting on the harsh rhetoric being bandied around, told Reuters: "In trying to maintain this entity called the Anglican Communion, Williams' job has become virtually impossible."

"Maybe this is the point when it is brought home to the Americans that this is not a pain-free option."