Boris Johnson is facing legal action after cancelling bus adverts paid for by evangelical Christian groups that promoted the idea that gay people can be converted from homosexuality.

The groups behind the "Not gay … and proud" adverts, which were pulled by the London mayor earlier this month, said they were likely to seek a judicial review of the mayor's decision on the grounds that it breached their rights to freedom of religion and freedom of expression as guaranteed under the European convention on human rights.

Anglican Mainstream and the Core Issues Trust, whose supporters advocate that gay Christians seek counselling and treatment to curb or even reverse homosexual instincts, said it might also take legal action for breach of contract against the advertising company that booked the adverts, CBS Outdoor.

"Since Boris Johnson intervened, there seems to be a much broader issue about freedom of speech at stake and that is weighing heavily upon us," said the Rev Lynda Rose, a spokeswoman for Anglican Mainstream. "We feel it is not right that people are not able to express legitimate views that are not an incitement to hatred."

The adverts were due to start running last week on London buses. The slogan, "Not gay! Ex-gay, post-gay and proud. Get over it!", was a response to an ongoing bus advertising campaign being run by the gay rights group Stonewall which states: "Some people are gay. Get over it." Within two hours of the Guardian revealing that the ads had been booked, they were cancelled by Johnson, who 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." His rivals in the campaign to become the next London mayor, Ken Livingstone and Brian Paddick, also condemned the adverts.

Critics of the evangelical groups have claimed that "reparative therapy" for gay Christians can cause considerable distress and stores up problems for people who suppress their homosexuality and enter into marriages or have children in heterosexual relationships. But the leader of the Core Issues Trust, Mike Davidson, has said: "Homoerotic behaviour is sinful."

The row over the adverts blew up during the government consultation on opening up marriage to same-sex couples, which continues until June, and Anglican Mainstream and the Core Issues Trust are set against the proposals. On Tuesday, they accused liberal bishops of imposing a "neo-pagan worldview" by supporting gay marriage and claiming there should be "a recognition of God's grace at work in same-sex partnerships".

On Saturday, a group of Church of England bishops wrote to the Times complaining that recent statements by church leaders had given the mistaken impression that the Anglican church was universally opposed to the extension of civil marriage to same-sex couples.

"The fact there are same-sex couples who want to embrace marriage should be a cause for rejoicing in the Christian church," said the letter, signed by the Very Rev Jeffrey John, dean of St Albans, the Right Rev Alan Wilson, bishop of Buckingham, and 13 other senior clergy and lay members of the General Synod.

In response, Anglican Mainstream and the Core Issues Trust issued a statement warning that the liberal clergy were trying to "unacceptably redefine Judeo-Christian belief".

"They do not have the standing either to rewrite or reinterpret the clear teaching of the Bible, which the Church has always understood to prohibit any and all sexual relations outside the union for life of one man and one woman," said Canon Dr Chris Sugden, executive secretary of Anglican Mainstream.