A Christian bakery will be fighting for its beliefs in a Belfast court today (26 March) after it refused to make a cake with a picture of Sesame Street puppets Bert and Ernie endorsing gay marriage.

The Ashers Baking Company in County Antrim cancelled an order for the cake showing the popular Sesame Street characters arm in arm with the slogan "Support Gay Marriage", and logo of gay rights group, QueerSpace, because it contravened the religious convictions of the firm's directors.

The Equality Commission is taking legal action today at the District Judges Court in Belfast against the firm on behalf of customer Gareth Lee, a gay activist, for allegedly breaching statutory duty not to discriminate on the grounds of sexual orientation in the provision of goods or services.

Defence lawyer Aidan O'Neill QC, who was commissioned by the Christian Institute that is supporting the bakery, previously told the Daily Telegraph if the case is upheld "it could undermine freedom of conscience".

The row has split public opinion in Northern Ireland. Members of the Democratic Unionist Party (DUP) have proposed to introduce a "conscience clause" into law to exempt businesses from discrimination law allowing them to refuse to provide services against their religion. Supporters say it would protect freedom of belief while opponents argue this would lead to discrimination against homosexuals.

Ashers general manager Daniel McArthur told the institute it was not the first time the company has refused customer orders which went against their beliefs, including those which contained pornographic images and foul language.

"If we don't stand up against this case how can we stand up further down the line?" he said.

The commission has been criticised for spending public money – said to be £33,000 ($49,000) – in seeking £500 damages from the bakery owners.

Thousands have also reportedly donated to a fundraising campaign, organised by the institute to support the bakery's legal fight.

Same-sex marriage is not legal in Northern Ireland, which is the only part of the UK not to have introduced the legislation.