A Christian-run bakery in Northern Ireland is facing legal action after refusing to produce a cake carrying a picture of the Sesame Street characters Bert and Ernie and the slogan “support gay marriage”.

Ashers Baking Co, based in Newtownabbey terminated the order saying that it went against the directors’ religious beliefs. Proprietors insisted that producing the cake with the slogan and a picture of the puppets arm in arm printed onto the icing, would amount to endorsing the campaign for the introduction of gay marriage in the province, and go against their religious convictions.

But the Equality Commission for Northern Ireland has now written to the firm insisting that are in breach of the law. It claimed that refusing to print the cake amounted to discrimination on grounds of sexual orientation against the man who placed the order.

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A letter signed by the legal office orders the firm to remedy “illegal discrimination” within seven days or be taken to court by the commission.

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The Christian institute, which is supporting the bakery, says it is not discriminatory for managers to refuse to endorse a political campaign. Unlike the British mainland, gay marriage is not legal in Northern Ireland.