The controversy was sparked when local gay rights organisation, QueerSpace, placed an order at the Belfast branch of Ashers Baking Company last May.

The proposed design of the cake included a picture of "Sesame Street" characters Bert and Ernie alongside QueerSpace's logo and the message "Support Gay Marriage". It was ordered for an event to mark the International Day Against Homophobia and Transphobia.

The Equality Commission for Northern Ireland said that it would be taking legal action against Ashers Baking Company and that the case has wider implications for the equality laws and discrimination in Northern Ireland.

"This case raises issues of public importance regarding the extent to which suppliers of goods and services can refuse service on grounds of sexual orientation, religious belief and political opinion," the commission said in a statement.

Daniel McArthur, general manager of the 22-year-old family-run business, said in a video published this week that the bakery would not be backing down and this was a "David and Goliath battle".

"Ashers Baking Company is willing to serve any customer regardless of their sexual lifestyle... but we don't want to be forced to promote a cause which is against our biblical beliefs," McArthur said in the video on the Christian Institute website.

Prominent Catholic priest Tim Bartlett, who was on the panel of Belfast's Pride festival this year, has sided with the bakery.

Bartlett told the BBC that he would not work with the gay community until "the right of all people, in this case Christians, to freedom of conscience is vindicated and respected by the Equality Commission and the gay community".

Northern Ireland is the only region of the United Kingdom that still prohibits gay marriage, which was legalised in England and Wales in July 2013, with Scotland following suit in February.

The Republic of Ireland will hold a referendum on same-sex marriage early next year.