Sinn Fein has vowed to torpedo an attempt by the DUP to amend equality laws to introduce a conscience clause for people with religious convictions.

First Minister Peter Robinson has backed his colleague Paul Givan's Private Member's Bill, which is aimed at allowing businesses to refuse services to a customer if they feel it is against their religious convictions.

But critics of the Bill have reacted angrily, claiming it is legalising discrimination in Northern Ireland. Last night a Sinn Fein spokesman said it was "opposed to any dilution of equality legislation", and accused the DUP of trying to "undermine equality and hard-won equality legislation".

This stance means the proposed Bill, which is expected to be brought before the Assembly next year, is unlikely to be passed.

The Freedom of Conscience Bill was prompted by the threat of legal action against a Christian bakery business. Ashers is facing a court battle over its decision to refuse to make a cake that carried a pro-gay marriage slogan.

The Equality Commission, which has taken the case against the business on behalf of the customer whose order was declined, has alleged the bakery's stance was in breach of legislation.

Mr Givan said he believed this was wrong. His proposed amendment to the law would mean that a Catholic adoption agency could refuse to place a child with a same-sex couple, but an evangelical green grocer couldn't refuse to sell an apple to a gay man.

He clarified that the difference was a service or act that could be seen as endorsing a same-sex sexual relationship. "The selling of apples would not involve the evangelical grocer being required to endorse, promote or facilitate a same-sex sexual relationship in violation of his/her faith identity so there is no conflict," he said.

However, a Catholic adoption agency "would not be required by law to choose between either being willing to act in violation of their faith by placing a child with a same-sex couple, thereby endorsing a same-sex union and same-sex parenting, or ceasing operations".

Mr Robinson backed the Bill and said: "I want to see a society in Northern Ireland which is tolerant of everyone's views. That tolerance must include provisions to ensure that those with deeply held religious views are protected." But the Rainbow Project, an advocracy group that gives a voice to the gay community, claimed the Bill was an attempt to "legalise discrimination against LGB (lesbian, gay, bisexual) people".

It said the Bill would mean restaurants could deny a same-sex couple a table, a mortgage provider could refuse to lend to a same-sex couple and hoteliers could deny a room to a same-sex couple - because all of these things could be regarded as "facilitating same-sex relations".

John O'Doherty, director of the Rainbow Project, said Stormont should be "focusing on publishing its long overdue sexual orientation strategy".

"The fact that this proposed legislation will only be used to discriminate against LGB people shows that it is not motivated by a desire to protect those of religious belief, but by a hostility to LGB people and their hard-won rights," he said.

"This is just updating 'No dogs, No blacks, No Irish' to include 'No gays'."

Ulster actor Gerard McCarthy blasted the proposed clause as "crazy" in the light of 20,000 schoolchildren contemplating suicide least year due to homophobic bullying.

"It appears that if you are Christian you are allowed not to abide by the equality laws," he said.

The launch of the Bill was held in the senate chamber in Stormont and was addressed by First Minister Mr Robinson and Peter Lynas from the Evangelical Alliance. It is set to be officially proposed and debated in the Assembly next year.

Sinn Fein has voiced opposition.

"We demand the enactment of the Bill of Rights as promised in the Good Friday Agreement," a spokesman said. "We will continue to support equality for everyone on this island regardless of colour, race, creed and sexual orientation. This conscience clause is another example of the DUP's desire to undermine equality and hard-won equality legislation."

Alliance MLA Stephen Farry urged caution: "Any general conscience clause would seriously undermine equality protections and facilitate unjustified discrimination in a wide range of scenarios, with serious implications for many people."

What does proposed bill mean?

Paul Givan says the focus of his Northern Ireland Freedom of Conscience Amendment Bill is “making space for rights that clash”. He says it aims to “make space for providers” and proposes that businesses may refuse a situation where they feel they are required to “endorse, promote or facilitate a same-sex sexual relationship in violation of his/her faith identity”.

For example, a Catholic adoption agency could refuse to place a child with a same-sex couple, a Muslim printer could refuse to print a book promoting same-sex sexual relationships and an evangelical photographer would not be required to choose between taking photographs at a civil partnership ceremony and losing their livelihood.

But, Mr Givan says, an evangelical grocer could not refuse to sell an apple to a gay man, or a Muslim printer could not refuse to print a brochure publicising coffee tables made by a lesbian cabinet maker.

The Rainbow Coalition, which supports gay rights, has claimed that under the Bill restaurants could deny a same-sex couple, a mortgage provider could deny a mortgage to a same-sex couple, and hoteliers could deny a room to a same sex couple as all could be seen as endorsing or facilitating same-sex relations.

The draft Bill has been published on the DUP website. The party is inviting responses about its contents to be submitted to its headquarters on Dundela Avenue in east Belfast. The consultation will close at 5pm on Friday, February 27.

Belfast Telegraph