Peter Tatchell, the gay human rights activist, has reiterated his support for Ashers bakery after it refused to produce a cake with a message in support of gay marriage as the case went to the Supreme Court in Northern Ireland today.

'Although I strongly disagree with Ashers opposition to marriage equality, in a free society neither they, nor anyone else, should be required by law to facilitate an idea to which they object,' Tatchell told Christian Today.

'While Christian bed and breakfast owners and civil partnership registrars were clearly wrong to deny service to gay people, the Ashers case is different. It is about the refusal to facilitate an idea – namely, support for same-sex marriage.

'The equality laws are intended to protect people – not ideas – against discrimination. A business providing a public service has a legal duty to do so without discrimination based on race, gender, age, faith, sexuality and so on.'

Tatchell's comments come as the highest court in the UK is sitting in Belfast today and tomorrow, considering an appeal in the protracted legal action against Ashers by the Equality Commission.

Ashers has mounted a vigorous defence, supported by the conservative pro-family campaign group, the Christian Institute.

The appeals comes after a county court judge decided the company had broken political and sexual orientation discrimination laws.

The judge ruled against Ashers, run by the McArthur family, who are Christians, for refusing to fulfil an order to make a £36.50 cake with a slogan supporting same-sex marriage because it conflicted with their religious beliefs.

The Christian baking company was found to have discriminated against Gareth Lee by refusing to bake him a cake with the slogan 'Support Gay Marriage'. The family-run business was taken to court by Lee, backed by the Equality Commission for Northern Ireland, and ordered to pay £500 in compensation – a decision that was later upheld by the Court of Appeal in Belfast in October 2016.

Now escalated to the UK's top court, the case has highlighted the tension between equality laws and religious freedom with a last-minute intervention by Northern Ireland attorney general suggesting the case could raise questions about the legitimacy of Northern Ireland's legislation.

John Larkin QC suggested the equalities laws used against the company may contravene Northern Ireland's commitment to the European Court of Human Rights. He said the case was 'about expression and whether it's lawful under Northern Ireland constitutional law for Ashers to be forced...to articulate or express or say a political message which is at variance with their political views and in particular their religious views.'

In 2016 Ashers lost its appeal at the Royal Court of Justice in Belfast with judges finding found 'this was direct discrimination' because it would not have objected to a slogan that supported heterosexual marriage.

The issue was Ashers 'would not provide a cake with a message supporting a right to marry for those of a particular sexual orientation', the court ruled.

The general manager of Ashers Baking Company, Daniel McArthur, said today that 'some people want the law to make us support something with which we disagree'.

Also speaking outside the court, Dr Michael Wardlow from the Equality Commission Northern Ireland said the case is about a business being accountable to 'settled laws' when offering services in public realm, not about quashing convictions or views.

Tatchell said: 'Most Christians in Britain support same-sex civil marriage. Ashers do not. They oppose gay marriage and anything that endorses it. They support discrimination in marriage law. However, in my understanding, discrimination is not a Christian value.

'The Appeal Court was wrong, in my view, to rule that the gay customer Gareth Lee, was discriminated against because of his sexual orientation. Ashers did not refuse to serve him. They simply refused to decorate his cake with a pro-gay message. That is their right in a democratic society.

'The Appeal Court verdict against Ashers Bakery was a defeat for freedom of expression. As well as meaning that Ashers can be legally forced to aid the promotion of same-sex marriage against their wishes, it also implies that gay bakers could be forced by law to decorate cakes with homophobic slogans. Discrimination against people should be illegal but not discrimination against ideas and opinions. I hope the Supreme Court will recognise this distinction and reverse the decision of the Appeal Court.'