Sir Ian McKellen, who played the wizard Gandalf in The Lord of the Rings trilogy, is to come out publicly in backing for the openly gay bishop at the centre of the Anglican Church’s split over homosexuality.

The actor will join Bishop Gene Robinson at a high-profile event to mark the launch of a new film about homosexuality and the Church on the eve of the Lambeth Conference in July.

A celebrity cast-list of 900 liberal clergy, actors and celebrities from the film world in Britain and the US are expected at the premiere of For the Bible Tells Me So at the South Bank in July.

“This evening will be a celebration of the lives and ministries of gay and lesbian people, on the eve of the Lambeth Conference of bishops in Canterbury,” Bishop Robinson said.

Some of the proceeds will go toward Aids work in Africa.

The event, just two days before the three-week Lambeth Conference begins at Kent University in Canterbury, will further derail the hopes of the Archbishop of Canterbury, Dr Rowan Williams, of keeping debate over gays low-profile.

Of more than 800 bishops invited from around the world, about a quarter have decided not to attend the conference. These are mainly from Global South provinces such as Nigeria. Nearly 300 bishops are instead going to the alternative Global Anglican Future Conference in Jordan and Israel in June, although some will be attending both.

Bishop Robinson has not been invited to the Lambeth Conference but will be in Canterbury anyway, attending and speaking at fringe events.