The head of the worldwide Anglican Communion has made it clear he is powerless to stop conservative Canadian and US congregations, upset with their national churches' positions on homosexuality, from leaving and affiliating with orthodox branches in Latin America and Africa.

It was a frank admission by Archbishop of Canterbury Dr Rowan Williams, leader of the world's 77 million Anglicans, of the limits of his power, even though he is opposed to cross-border ecclesiastical moves.

Dr Williams was responding to a plea by the liberal leadership of the Anglican Church of Canada to address the fact that the orthodox Anglican Church of the Southern Cone of the Americas had started in November giving oversight to some congregations - or "intervening" in Canada.

"I have no canonical authority to prevent these things, but I would simply repeat what was said in my advent letter (in December), to the effect that I cannot support or sanction such actions," Dr Williams wrote the Canadian archbishops.

The Anglican Church of Canada released the letter on Monday. Dr Williams did not specifically address the Episcopal Church of the United States but the principle would apply there too since the situation is directly parallel.

In both the United States and Canada, African and Latin American branches of the Anglican Communion have taken conservative churches under their wings. In December, the entire Californian Diocese of San Joaquin defected and joined the Southern Cone church, based in Argentina.

So far, the number of members who have left the mainline Canadian and US branches is small but more are considering seceding.

It is a sign of the difficulty that Dr Williams will have in keeping the members of the worldwide Anglican Communion together as the liberals and conservatives argue over the issue of homosexuality and scriptural authority.

In Canada, the Diocese of New Westminster, British Columbia, has allowed the blessing of homosexual marriages, and three other dioceses have advocated gay marriage ceremonies.

Same-sex marriage is legal in Canada, though religious organisations are not required to perform the ceremonies.

In the US, the Episcopal Church has allowed the ordination of an openly gay bishop, and the blessing of gay unions is also practiced in some congregations.