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If an equalities watchdog wins a legal case against a Christian bakery that refused to bake a cake decorated with gay marriage slogans, the decision could force a Muslim printer to publish cartoons of the Prophet Mohammed, a lawyer claims.

The ruling could also mean a T-shirt company owned by lesbians would be obliged to print shirts with slogans describing gay marriage as an "abomination."

Leading human rights lawyer Aidan O'Neill QC said the implications of the decision concerning Ashers Baking Company would have far-reaching consequences for businesses that refuse custom based on principles.

O'Neill's legal opinion was commissioned by the Christian Institute, which is supporting the bakery's defense in a case due before a court in Belfast later this month.

Ashers, based in Newtownabbey, Northern Ireland, last year canceled an order for a cake from LGBT activist Gareth Lee featuring Sesame Street characters Bert and Ernie and the slogan "support gay marriage."

Northern Ireland is the only part of the UK where same-sex marriage is not yet legal.

The owners of the company said the order, which also featured the logo of gay rights group QueerSpace, went against their religious convictions as Christians.

Following a complaint, the Equality Commission for Northern Ireland wrote to Ashers claiming it is breaking the law by discriminating on the grounds of sexual orientation.

The bakery was ordered to "remedy your illegal discrimination" within seven days or be taken to court.

A spokesman for the Rainbow Project, a gay rights group affiliated with QueerSpace, told RT at the time the bakery was wrong to pick and choose its customers.

"The Equality Commission have made a very reasoned application of the law and were right to say legally the bakery was wrong to deny service to this customer," the spokesman said.

"We're in an untenable situation here where a Northern Irish gay couple can go anywhere else in the UK and get married, but when they return home their marriage is reclassified as a civil partnership without their consent. This throws up all sorts of constitutional issues for the UK," he added.

Lee is seeking a small amount of compensation and an admission that the bakery's treatment amounted to discrimination, based on employment and equality law.

In his legal opinion, O'Neill warned the Commission's case does not take human rights protections into account, such as freedom of conscience.

He also argued it ignores the "negative right of expression," which the European Court of Human Rights ruled is contained within Article 11 of the European Convention on Human Rights.

O'Neill argued this "negative right," which protects a person's right not to express support for an opinion or political position, has historical precedent.

He claims it was upheld by Sir Thomas More when he refused to sign the Act of Supremacy declaring Henry VIII to be Head of the Church of England, thereby superseding the Pope, in 1530.

O'Neill also claimed there would be no legal defense for "a Muslim printer refusing a contract requiring the printing of cartoons of the Prophet Mohammed."

Colin Hart, director of the Christian Institute, said "This is a truly alarming case with far-reaching implications for freedom of speech. It's wrong for the law to force people to say things they don't believe."

"Ashers serve gay customers all the time. But they didn't want to promote gay marriage," he added.