It was the moment that conservative Christian groups' growing stridency in the British political arena went too far, at least for Boris Johnson. On Thursday evening the London mayor gave the clearest sign yet that radical religion and politics still do not mix in the UK when he slapped an almost instant ban on a planned bus advertising campaign that promoted Christian groups' belief in a cure for homosexuality. Whether it was the electoral sensitivities of May's mayoral poll or the feeling that the adverts, due to run on around two dozen red London buses, were underpinned by homophobic sentiments, Johnson acted within an hour of news of the campaign leaking out to draw a line in the sand. His move will startle conservative Christians who have been agitating to replicate in British politics the American example where religious values take centre stage in campaigning.

Neither Chris Bryant, the prominent gay Labour MP, nor Ben Summerskill, the director of the gay rights group Stonewall, wanted the advert to be pulled, both citing freedom of speech.

The ad campaign was meant to show Christian groups taking on their opponents in the most public arena possible. Anyone standing by St Paul's Cathedral would have been able to see the clash of ideas roll past in what would have been a battle of the buses. Stonewall was already running ads on 1,000 buses in the capital, including the number 76, which runs past Sir Christopher Wren's masterpiece. Their ads say: "Some people are gay. Get over it!" The Christian groups Anglican Mainstream and the Core Issues Trust, which encourage gay people to consider changing their sexuality through therapy, wanted to challenge that directly with ads on different buses on the same route. Their banner was to be in the same format but saying: "Not gay! Ex-gay, post-gay and proud. Get over it!" It was, anyway, never the clearest piece of public communication. "Post-gay" and "ex-gay" are terms used in some Christian circles that refer to homosexual people who have undergone pastoral and spiritual therapy to change their sexual preferences.

The political context for the tit-for-tat ad booking is the ongoing and highly charged Home Office consultation on whether to extend marriage rights to gay people, and the Christian groups' ad sparked outrage among supporters of gay rights who believed it was implicitly homophobic. In a heavy blow to conservative Christians' increasing desire to mix with other campaign groups in the political arena, the mayor's office said the ads were inconsistent "with Transport for London's commitment to a tolerant and inclusive London".

The attempt by Christian groups to take their beliefs into the hurly burly of the public and political arena was no aberration. On the highly charged issues of abortion, sexuality or even the fundamental question of whether there is a God at all, some Christian factions have decided the time has come to project their beliefs beyond their private prayers. Catholic groups have started protesting outside abortion clinics and Christian politicians have become increasingly confident in voicing their views. Observers of Christian politics have noticed that tactics previously seen only in the high-octane arena of American religious affairs are beginning to take hold in the UK.

Last year the Christian party took on atheists who sponsored ads that read: "There is probably no god. So stop worrying and enjoy your life". The Christian riposte read: "There definitely is a God. So join the Christian party and enjoy your life."

"There has been a trend among conservative evangelical groups to take a more political stance," said Simon Barrow, co-director of Ekklesia, a social affairs thinktank that has charted the rise of what it calls "aggressive Christian conservatism". "In the past, piety and politics were seen as distinct and even in tension with one another. But the scale of their political power has been in long-term decline and some groups of Christians have reacted with fear and anger. They fear they will become irrelevant and they are angry because they feel they hold the truth and have a right to be at the centre of the arena."

In the arena of gay rights, Summerskill, chief executive of Stonewall, has seen the trend develop as the coalition government consults on whether marriage should be extended to gay people. The Home Office consultation was launched in March and runs until 14 June and the debate has riven the Church of England, with 100 priests signing a letter demanding the right to conduct gay marriages in their churches. That, says Summerskill, is the highly charged political context which explains why the orthodox Christian groups have decided to run their ad campaign now.

"There is an aspiration among some of the Christian organisations in Britain to raise the temperature in the way it is raised in America," he said. "That is why we have worked so hard with the Conservative party to try to reduce the effect of that. When you talk to the Christian groups privately they look at America and they are slightly envious of what they see as very high-octane public arguments over the issues they feel strongly about. If there is a debate to be had about extending marriage to gay people it should be around the public policy benefits. It shouldn't be around a first- or second-century interpretation of the Bible."

Members of Anglican Mainstream hold strong views about homosexuality that are not made clear by the bus ad. Asked to explain how a gay Christian would be treated within a church attended by supporters of Anglican Mainstream, the movement's UK spokeswoman, the Rev Lynda Rose, drew an analogy with people with drinking problems.

"If someone is an alcoholic and comes to church and says they are happy with the way they are, others will say that's not the way to go," she said. "They are still free to say: 'No thank you,' but would that be right?"

In 2009 Anglican Mainstream ran a conference called Sex in the City, which was billed as "ideal for clergy, rabbis, psychologists, therapists, educators and others concerned about the plethora of sexual issues confronting us in today's society, including mentoring the sexually broken, the sexualisation of culture, pornography, the Bible and sex, and marriage, the family and sex". It promised "a special focus on how religious professionals and friends/relatives can respond biblically and pastorally to those struggling with unwanted same-sex attraction". An American speaker, Joseph Nicolosi, ran a session on "shame, homosexuality and the practical work of reparative therapy."

Christian anti-abortion campaigners are also taking their fight into the public arena in the UK, buying in American expertise.

Catholic activists from a British branch of a US group called 40 Days for Life have been holding daily prayer vigils outside abortion clinics. They marked a change of tactics and were part of the reason that earlier this year the British Pregnancy Advisory Service warned that people would be put off from becoming involved in abortion services.

Robert Colquhoun, 40 Days for Life's campaign director in the UK, set up the British operation in 2010 after seeing the tactics in action in Ottowa in Canada. He said he was impressed by the way vigils brought the ethical and spiritual issues out of the abstract and into the communities where abortion clinics operate, and how the campaign had claimed responsibility for the closure of 22 abortion clinics in the US so far. Affiliated campaigns in 13 countries pay a $199 fee to join up, he said, while campaign training is carried out centrally from an office just outside Washington DC, with sessions carried out via broadcasts over the internet.

"In the UK, campaigns on ethical issues have been too focused on politics and have been missing the mark," he said. "Whether euthanasia, gay marriage or abortion, it used to be a question in the UK of encouraging people to write to your MP and that used to be it. But there is so much more to these issues and we focus on bringing it to life in the community. Many people are pro-life but it has been an abstract idea for them, but by having a campaign you can get people involved through personal participation. If you don't have something going on at the grassroots, what chance do you have in parliament and the courts?"