Boris Johnson, the incumbent mayor of London, has said a series of ‘ex-gay’ bus adverts which he pulled last week would have caused ‘great offence’ and prompted an ‘intense backlash’ against Christians in the capital.

Mr Johnson was attending a debate yesterday ahead of the 3 May mayoral elections organised by the London Church Leaders, Faith to Engage, and the Evangelical Alliance.

The Evening Standard said he told the audience: “The job of mayor is to unite, to stop prejudice.

“The backlash would be so intense it would not have been in the interest of Christian people in this city.”

Ken Livingstone agreed with Mr Johnson’s decision to pull the adverts, saying: “In my view Boris was right to pull them”.

The adverts were to be funded by ‘gay cure’ therapy advocates Anglican Mainstream and the Core Issues Trust and were banned at the mayor’s intervention last week.

They said: “Not gay! Post-gay, ex-gay and proud. Get over it!” and were a direct response to Stonewall’s “Some people are gay. Get over it!” campaign.

Intervening to ban the adverts, Boris Johnson said last Thursday evening: “London is one of the most tolerant cities in the world and intolerant of intolerance. It is clearly offensive to suggest that being gay is an illness that someone recovers from and I am not prepared to have that suggestion driven around London on our buses.”

Co-organisers of yesterday’s mayoral debate, the Evangelical Alliance, have opposed gay rights in the past but were not involved with the ex-gay bus adverts.

The group’s head of public affairs, Don Horrocks, has said equal marriage rights for gay couples would soon lead to people “wanting to marry their horse” and is a director of the limited company behind the Coalition for Marriage, which the Evangelical Alliance promotes on its website.

The Coalition for Marriage recently confirmed to the Independent that the Evangelical Alliance had given it “seed money”.

A national advertising campaign has pushed signatures on the petition to block marriage for gay couples to nearly 450,000. PinkNews.co.uk is supporting the Coalition for Equal Marriage, set up in opposition to the anti-marriage campaign, which has 46,000 signatures.

Separately, the ASA responded to complaints today saying it would not be investigating the ex-gay bus adverts as they were no longer scheduled to appear and stressed that it does not clear advertisements before they run.