The Court of Appeal has begun hearing the case of Christian charity prevented from advertising anti-gay posters on 24 buses in London.

Paul Diamond, for Core Issues Trust, told three appeal judges at the heart of the case was the “ironical” situation in modern British society where ancient Biblical scriptures, which played an important role in forming the nation’s morals, were now in danger of containing views which could no longer be expressed “in a land with a reputation for free speech”.

Mr Diamond said the Bible and its Christian scriptures only permitted sexual relationships between one man and one woman in marriage and people should be entitled to express that view.

He said the case raised the question: “Is the belief that homosexuality is a sin worthy of respect in a democratic society?”

“We were simply trying to engender a debate. There is a debate out there – it is not all one way. People don’t have the same views. There is an alternative,” he said.

In April 2012, Core Issues Trust tried unsuccessfully to advertise anti-gay posters on 24 London buses in a direct response to Stonewall’s pro-gay posters.

The Trust’s posters read: “Not Gay. Ex-Gay, Post-Gay and Proud. Get over it!”

Although the ad campaign was passed by the Committee of Advertising Practice, Mayor of London Boris Johnson instructed Transport for London (TfL) to pull the campaign before it could run – just days before the 2012 Mayoral Election.

According to PA, the Trust accuses Mr Johnson of unlawfully using his position as chairman of TfL to obtain the ban in order to secure the “gay vote” and advance his 2012 re-election campaign.

Core Issues Trust attempted to challenge the TfL decision – but in March the High Court declared it was not unlawful.

However, Mrs Justice Lang ruled that TfL’s process in introducing the ban “was procedurally unfair, in breach of its own procedures and demonstrated a failure to consider the relevant issues”.

But that was outweighed by factors against allowing the ad, including that it would “cause grave offence” to those who were gay and was perceived as homophobic, “thus increasing the risk of prejudice and homophobic attacks”.

Mr Diamond is arguing on appeal that an “email trail” not disclosed by the Mayor’s office at the High Court, and discovered later following a Freedom of Information request by Core Issues Trust, supported its claim over Mr Johnson’s role in banning the ad.

The Trust says its works with gay people seeking to change their lifestyles, but rejects the idea of offering a gay cure.

It is asking the appeal judges to uphold its right to freedom of expression under Article 10 of the European Convention on Human Rights, which it says has been violated by the ban.

It is also relying on Article 9, which protects the right to freedom of thought, conscience and religion.

The case continues.