London buses have been booked to carry a Christian advertising campaign expected to start next week, which asserts the power of therapy to change the sexual orientation of gay people.

The full length advert, which will appear on five different routes in the capital, is backed by the Core Issues Trust whose leader, Mike Davies, believes "homoerotic behaviour is sinful". His charity funds "reparative therapy" for gay Christians who believe that they have homosexual feelings but want to become straight. The campaign is also backed by Anglican Mainstream, an worldwide orthodox Anglican group whose supporters have equated homosexuality with alcoholism.

The advert will say: "Not gay! Post-gay, ex-gay and proud. Get over it!" Post-gay and ex-gay are terms used by some Christians, psychotherapists and psychiatrists to refer to homosexual people who have undergone spiritual or pastoral therapy and, according to an Anglican Mainstream definition, have "now left a homosexual lifestyle [and experienced] an increased emotional and sexual attraction to the opposite biological gender and possibly a reduction in or loss of same-sex attraction."

The buses are due to roll out on Monday morning on some of the most popular routes. They will be seen for two weeks travelling past St Paul's Cathedral, down Oxford Street, round Trafalgar Square and through Piccadilly Circus as well as across other parts of the capital.

The campaign is an explicit attempt to hit back at gay rights group Stonewall, which ran its own bus advert saying: "Some people are gay. Get over it." The Christian groups have used the same black, red and white colour scheme as Stonewall and accuses it of promoting the "false idea that there is indisputable scientific evidence that people are born gay".

The Rev Lynda Rose, a spokesperson for the UK branch of Anglican Mainstream said because her group adheres to scripture that all fornication outside marriage is prohibited, it believes that homosexuals are "not being fully the people God intended us to be".

It has sparked an angry response from gay rights campaigners.

The former Europe minister and former Anglican curate, Chris Bryant MP, said the advert was cruel, particularly to teenagers struggling to come to terms with their sexuality, for promoting the idea that you could become "ex-gay".

"The emotional damage that is done to the individuals who try to suppress their sexuality, the women they marry and the children they might have is immeasurable," he said. "Most sane Christians believe that homosexuality is not a lifestyle or a choice but is a fact to be discovered or not. The pretence that homosexuality is something you can be weaned off in some way is a fundamental misunderstanding of the nature of creation."

Ben Summerskill, the chief executive of Stonewall, said the adverts were clearly homophobic and added: "The only reason some gay people might want to stop being gay is because of the prejudice of the people who are publishing the ad.

"The promotion of this voodoo therapy is hugely irresponsible given the damage that it appears to do to some people."

Attempts to "treat" or alter sexual orientation have been strongly condemned by leading medical organisations. The Royal College of Psychiatrists has warned that "so-called treatments of homosexuality create a setting in which prejudice and discrimination flourish" and concluded in 2010 that "there is no sound evidence that sexual orientation can be changed". The British Medical Association has also attacked "conversion therapy", a related field to reparation therapy, passing a motion asserting that it is "discredited and harmful to those 'treated' ".

Rose said therapies endorsed by Anglican Mainstream and Core Issues were not coercive and stressed that they are appropriate for people who want to change their sexual attractions, for example if they are married and are worried about the impact of a gay lifestyle on their children.

"Reparative therapy works to help men, who want to change their sexual orientation, naturally dissipate their homoerotic feelings and maximise their heterosexual potential," she said.

In a statement, Anglican Mainstream and Core Issues said Stonewall's slogan is "merely another attempt to close down the critical debate about being gay, and marriage 'equality' ". They accused Stonewall of riding roughshod over individuals who chose to "move out of homosexuality".

The statement continued: "Both organisations recognise the rights of individuals to identify as gay, and to live according to their own values. But by the same token, they believe individuals – such as married men and women unhappy with their homosexuality – should be supported in developing their heterosexual potential where this is the appropriate life choice for them … Current scientific research says there is no gay gene and that sexuality is far more fluid than has hitherto been thought."

Assuming that the campaign prompts complaints to the Advertising Standards Authority it is likely to be investigated against the advertising code rules relating to harm and offence and potentially to truthfulness and substantiation.

According to the code an advertisement must not contain content that is likely to cause serious or widespread offence. However the fact that a campaign is offensive to some people does not mean it will necessarily be banned.

"Particular care must be taken to avoid causing offence on the grounds of race, religion, gender, sexual orientation, disability or age," rule 4.1 of the code states. "Compliance will be judged on the context, medium, audience, product and prevailing standards"

The campaign could also prompt complaints about whether or not the statements that relate to "reparative therapy" – "post-gay, ex-gay" – can be substantiated by the Christian group.

• This article was amended on 16 April 2012. The original described Chris Bryant as an ex-vicar, when he is a former Anglican curate. A phrase that said post-gay and ex-gay were terms used by "Christians and some psychotherapists and psychiatrists" was also amended.