Simon Calvert, spokesperson for The Christian Institute, told the Belfast Telegraph: “So far, the case has generated legal costs on both sides which, combined, have broken the £100,000 barrier.”

In May 2015, Ashers was ordered to pay out £500 after refusing to make a sponge cake with a slogan promoting same-sex marriage.

The company was taken to court by LGBT rights activist Gareth Lee, backed by the Equality Commission for Northern Ireland.

The company is currently appealing the decision on the basis of article 10 of the European Convention of Human Rights, which guarantees freedom of expression.

Mr Calvert said the McArthur family, who run the bakery, had accumulated costs of over £50,000 and believed the Equality Commission’s costs “are at least as high”.

The appeal is due to begin on Monday 9 May after a last-minute intervention by Attorney General John Larkin QC in February caused a three-month adjournment.

Mr Larkin has been given permission by the appeal judges to make representations at the full hearing in May.