A court appeal launched by Christian-run Ashers Bakery over the so-called "gay cake case" is due to begin today.

Daniel McArthur, general manager of Ashers, arrived at the High Court in Belfast this morning with his wife Amy to begin a two-day hearing. The couple are seeking to overturn a decision against them over their refusal to produce a cake with a "support gay marriage" slogan for LGBT activist Gareth Lee.

They were fined £500 after Judge Brownlie ruled their decision amounted to discrimination under laws which prohibit the refusal of services on the grounds of religion, race, or sexual orientation.

"Ashers does not discriminate against anyone," Daniel McArthur told journalists outside the court this morning.

"We took issue with the message on the cake not the customer. As a family we believe we should retain the freedom to decline business that would require us to promote a cause with which we disagree."

However the chief commissioner of the Equalities Commission, which launched the legal challenge on behalf of Lee, said Ashers Bakery had been discriminatory.

"One of the things the Equality Commission does is support people who we believe have been discriminated against," said Dr Michael Wardlow this morning. "When it raises a strategic issue, that allows us to challenge discriminatory practice and raise awareness of the legislation.

"In this particular case we believe this has happened. We hope today the three judges will agree and Judge Brownlie's ruling will stand."

The case was highlighted after prominent LGBT rights activist, Peter Tatchell, announced his support for the McArthurs' right to free speech.

"Much as I wish to defend the gay community, I also want to defend freedom of conscience, expression and religion," he wrote in an article for the Guardian.

A petition supporting the McArthurs was launched by their legal backers, the Christian Institute, in January and now has over 9,000 signatories.

The appeal will likely be heard for two days with a ruling delivered on Thursday.