Ashers Bakery's 'gay cake' case will hit the Supreme Court next week as the UK's top judges make a historic first visit to Belfast.

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.'

The case will be heard on Tuesday and Wednesday next week, almost four years to the day after they first refused to bake a cake for Lee. Ashers' general manager, Daniel McArthur, said the decision was nothing to do with Lee's sexuality but because of the slogan supporting gay marriage he wanted on the cake.

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.

'We accept that it was the use of the word 'gay' in the context of the message which prevented the order from being fulfilled,' they said.

'This was a case of association with the gay and bisexual community and the protected personal characteristic was the sexual orientation of that community.'

The judges added: 'The fact that a baker provides a cake for a particular team or portrays witches on a Halloween cake does not indicate any support for either.'

But McArthur said the statement they were asked to print was 'totally contrary to our Christian beliefs'.

'Having the Supreme Court in Belfast hearing our arguments shows they recognise the seriousness of the issues at stake and that is an encouragement to us,' he said.

Simon Calvert, spokesman for The Christian Institute, a conservative Christian charity supporting the family, said: 'We believe the rulings in the lower courts undermine democratic freedom, religious freedom and free speech. It is not right to compel people to help make statements, whether in ink or in icing, with which they profoundly disagree.

'There are important principles at stake in this case which affect everyone. A truly tolerant society must allow for differences of belief.'