Freedom of speech concerns are being raised after Boris Johnson banned an anti-gay bus advert.
The advertisement suggesting that homosexuality could be cured, a response to Stonewall's 'some people are gay - get over it!' campaign, was to appear on London buses after being passed by the Committee of Advertising Practice.
But London mayor Mr Johnson stepped in within hours of the slogan becoming public. It stated: "Not gay! Ex-gay, post-gay and proud. Get over it!"
The Core Issues Trust, which raised around £10,000 to pay for the adverts alongside the Anglican Mainstream group, concentrates on "sexual and relational brokenness".
Its leader Mike Davidson told the Guardian newspaper: "I didn't realise censorship was in place. We went through the correct channels and we were encouraged by the bus company to go through their procedures. They okayed it and now it has been pulled."
Mr Johnson said: "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."
The incumbent mayor, who is standing for re-election on May 3rd, was accused of not having acted soon enough by his main challenger, Labour's Ken Livingstone.
"London is going backwards under a Tory leadership that should have made these advertisements impossible," Mr Livingstone said.
"The ads should never have been agreed full stop. If elected I will once more overhaul the rules on advertising on London's transport system so that this can never happen again, and ensure that we revive the progress we made under Labour after the retreat under a Tory administration."
Blogger Archbishop Cranmer issued a defence of the advertisements which was posted on the Anglican Mainstream website.
He argued: "Christians are called to proclaim the Good News, and whether that vocation be in a pulpit, upon a television screen, or walking up and down Oxford Street with a sandwich board, it ought to be tolerated in a free society."
The 2012 mayoral race has not seen sexuality issues feature prominently until now. Mr Johnson was keen to demonstrate his liberal views on the topic during the 2008 race after facing accusations of homophobia from some quarters.
Attitudes to homosexuality are currently the topic of debate nationally, however, as the coalition pushes ahead with its plans to legalise gay marriage.
A 12-week consultation is currently underway despite protests from religious leaders including Cardinal Keith O'Brien, the senior British Catholic, who called the proposals "grotesque".