Four British Christians who claim they lost their jobs as a result of discrimination against their beliefs are taking their cases to the European Court of Human Rights.
They include an airline worker stopped from wearing a cross and a registrar who did not want to marry gay couples.
All four lost separate employment tribunals relating to their beliefs.
Secular critics have said any ruling in favour of the group could “seriously undermine” UK equality law.
A ruling is not expected from the European court for several weeks.
The cases involve British Airways check-in clerk Nadia Eweida, nurse Shirley Chaplin, relationship counsellor Gary McFarlane and registrar Lilian Ladele:
- Ms Eweida, a Pentecostal Christian from Twickenham, south-west London, was sent home by her employer British Airways in 2006 after refusing to remove a necklace with a cross
- Devon-based nurse Mrs Chaplin was moved to a desk job by Royal Devon and Exeter NHS Trust Hospital for similar reasons
- Mr McFarlane, a Bristol counsellor, was sacked by Relate after saying on a training course he might have had a conscientious objection to giving sex therapy advice to gay couples
- Miss Ladele was disciplined after she refused to conduct same-sex civil partnership ceremonies in north London
Each individual had made a separate application to the court, but the cases are being heard together.
Court documents explain that Ms Eweida and Mrs Chaplin believe the UK law has “failed adequately to protect their right to manifest their religion” which is contrary to Article 9 of the European Convention on Human Rights.
This article provides a right to freedom of religion, including to worship, teaching, practice and observe elements of their faith.
They also claim that previous tribunal rulings have breached Article 14 of the convention, which outlaws discrimination based on religion.
Miss Ladele also believes her right to an “effective remedy” was infringed, and Mr McFarlane claims his right to a fair trial and right to a private life in the UK were breached.
Earlier this year, the UK’s equality watchdog, the Equality and Human Rights Commission, said the UK tribunals had come to the correct conclusion in the cases of Miss Ladele and Mr McFarlane.
But it conceded the the courts “may not have given sufficient weight” to Article 9 of the European convention in the cases of Ms Eweida and Mrs Chaplin.
Andrew Marsh, campaign director at religious group Christian Concern, whose sister organisation Christian Legal Centre is supporting Mrs Chaplin and Mr McFarlane, told the BBC the four could have had their beliefs respected by their employers without adversely affecting the people they serve.
“The crucial question in these cases is this: could these four individuals have been reasonably accommodated and their Christian faith respected, without detriment or damage to the rights of others – and the answer to that question is clearly yes.
“Each of them could have been reasonably accommodated without there ever being any danger of risk, significant risk to others or indeed of anyone who is entitled to a service being denied that service.”
However, the National Secular Society – which campaigns against “religious privilege” – said a European court ruling in favour of the quartet would undermine UK equality law.
The society’s director, Keith Porteous Wood, said the group was fighting the action: “We think that if it goes the wrong way it will cause a hierarchy of right, with religion at the top, and it’s going to be bad news for employers and for gay people.”
During Mrs Chaplin’s case, the NHS trust said the necklace her cross was on had breached health and safety guidelines. She lost her discrimination case in 2010.
Meanwhile, Miss Eweida, who was suspended by British Airways for breaching its uniform code, also lost her case against discrimination in 2010.
The airline changed its policy in 2007 to allow staff to display a faith or charity symbol with the uniform.
Mr McFarlane, a Christian marriage guidance counsellor from Bristol, lost his 2010 court bid to challenge his sacking for saying he might not be able to give sex therapy to homosexuals.
And, also in 2010, the Supreme Court ruled against Miss Ladele, who was disciplined by Islington Council for refusing to conduct same-sex civil partnerships.
The court refused her bid to challenge an appeal tribunal which overturned a previous decision by an employment tribunal that the council had discriminated against her.