The man at the centre of the 'gay cake' case is bearing a "heavy burden" because of the legal battle, his lawyer has told the UK Supreme Court sitting in Belfast, which has reserved judgement in the case.

The Christian owners of Ashers Bakery were found to have discriminated by refusing to make a product iced with the slogan Support Gay Marriage.

The order was placed by gay rights activist Gareth Lee in May 2014 and has been the subject of an appeal in Belfast.

His barrister Robin Allen QC said: "This is what was a relatively small incident in his life which has become enormously significant and continues to be so.

"That is a heavy burden to bear for one individual."

The original case against the McArthur family, who own Ashers and enjoy the backing of the Christian Institute lobby group, was taken by Mr Lee with the support of Northern Ireland's Equality Commission.

The bakers lost in the lower courts.

Mr Allen said the Commission was seeking "legal certainty" and stepped in because only a millionaire could afford to litigate this case.

"In the context of this case it is super-difficult because in relation to the provision of goods, facilities and services you are talking about a prohibition on discrimination on transactions which may range from a box of matches up to a long-term contract.

"In many cases, particularly true in a divided community, prohibition in relation to goods and services is a task which is difficult to police through a commission or litigation".

Northern Ireland's attorney general John Larkin QC has said an earlier Court of Appeal ruling on Ashers was wrong.

He said: "Mr Lee does not have the right to compel Ashers to express a message to deliver content which is inconsistent with their religious beliefs".

He added the McArthur family could avail of the European Convention on Human Rights to refuse to express a political opinion which was contrary to their religious beliefs, warning there was no objective or reasonable justification for interference.

He contended nothing in Northern Ireland constitutional law forced a choice between being the courier or conduit of a message to which they objected on religious grounds or exposure to civil liability in the courts.

A lawyer representing Daniel and Amy McArthur, who own Ashers, argued that the state is penalising the Belfast baking firm, with the courts effectively forcing them to make a cake whose message they disagree with as a matter of religious conscience.

The Supreme Court has reserved judgement, with a ruling not expected until early next year.

Its President Lady Hale said people should not expect the answer any time soon.

She said: "These cases are by definition complex and difficult and there are five people who have to make up their minds. That in itself takes more time than simply a jury verdict at the end of a long trial".