Christian group Anglican Mainstream is taking legal action against London mayor Boris Johnson after he banned bus advertisements that suggested thatv gay people could be cured.

The controversial posters, from Cure Issues Trust, proclaimed: "Not gay! Post-gay, ex-gay and proud. Get over it!" It mimicked an existing campaign by pro-gay group Stonewall which said: "Some people are gay. Get over it." Those advertisements have been accepted.

The "Not gay" adverts were backed by Anglican Mainstream, a worldwide orthodox Anglican group whose supporters have equated homosexuality with alcoholism and believe in traditional biblical teachings on marriage and sexuality.

It is understood that the organisation has instructed solicitors to proceed with legal action after the campaign was banned.

Johnson ordered the posters to be pulled just a few hours after the plans were made public. He labelled the campaign homophobic.

Johnson, who is standing for re-election on 3 May, said: "London is one of the most tolerant cities in the world and intolerant of intolerance.

"It is clearly offensive to suggest being gay is an illness someone recovers from and I am not prepared to have that suggestion driven around London on our buses."

The campaign, which was cleared by the Advertising Standards Authority (ASA), claimed therapy could change sexual orientation.

Boris Johnson Pulls Core Issues' 'Gay Cure' Posters from London Buses