The Christian owners of the Belfast-based Ashers bakery are being forced to act against their consciences and religious beliefs, the supreme court was told today.

The bakery, which is supported financially by the conservative, pro-family group The Christian Institute, is appealing against a £500 damages award made against it for refusing to produce a cake promoting gay marriage ordered by a LGBT activist in 2014.

David Scoffield QC, who represents the bakery's owners Daniel and Amy McArthur, told the supreme court: 'This is a case of forced or compelled speech, unlike other cases which have come before the court.

'Mr and Mrs McArthur have been penalised by the state in the form of the judgment at the county court for failing, through their family company, to create and provide a product bearing an explicit slogan – "Support Gay Marriage" – to which they have a genuine objection in conscience.

'The result of the approach taken by the district judge and court of appeal ... is that someone providing bespoke goods must provide goods at the request of the customer, provided that is not unlawful – however offensive.'

Scoffield added that the case raised an issue of principle since those with deeply held religious beliefs could be compelled to act against them. He said the McArthurs were being forced to 'choose between operating their businesses or living and acting in accordance with their religious beliefs, and we say that cannot be the law'.

Same-sex marriage is not legally recognised in Northern Ireland.

Gareth Lee had ordered the cake for an event to mark International Day Against Homophobia.

When Ashers refused to bake the cake, Lee sued, and his case has been supported by the Equality Commission for Northern Ireland.

In the county court case, the district judge Isobel Brownlie ruled that religious beliefs could not dictate the law. She ordered the firm to pay damages.

The court of appeal in Belfast upheld that ruling in 2016, after Ashers contended that it never had an issue with Lee's sexuality – rather the message he wanted on the cake.

Peter Tatchell, the gay human rights activist, defended the bakery. He told Christian Today: '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.'

The hearing continues.