A Christian woman has asked the Court of Appeal for a landmark ruling that she has the legal right not to be forced by an employer to work on Sundays.
Children's care worker Celestina Mba, from London, says she should be free to obey the Fourth Commandment in the Bible and rest and pray on the Sabbath.
Ms Mba, 58, a Baptist Christian and mother-of-three,said as she came to court: "I have never worked Sunday in my life.
"What I am hoping to achieve is to have my individual rights to worship recognised - remove my belief and there is no 'me'."
Mrs Mba, 58, wants three appeal judges to overturn an employment appeal tribunal (EAT) decision dismissing her claim for constructive dismissal against the London Borough of Merton.
She says she was working with autistic children at a respite centre run by the borough on the understanding that she would not have to work on Sundays, but after a time her employers reneged on the agreement.
She said she told her bosses before accepting the job that keeping Sunday as a day of rest and prayer was an issue of faith.
For two years, her beliefs were accommodated, but then she was told she would have to work full weekend shifts and was left with no option but to resign.
Her claim for constructive dismissal failed before the EAT partly on the grounds that observing the Sabbath was not a "core component" of the Christian faith.
The tribunal took into account a statement submitted to support her case by a senior clergyman, which stated only "some" Christians observe the Sabbath.
Michael Nazir-Ali, former Bishop of Rochester, wrote: "Some Christians will not work on the Sabbath (except for mercies), others may work only in an emergency; some Christians will want to wear a cross to manifest their faith, others will manifest their faith in some other way."
Mrs Mba's counsel, Paul Diamond, argued the EAT had misconstrued her belief and wrongly interpreted the fact that "some" Christians will not work on the Sabbath in a way that interfered with her individual right under Article 9 of the European Convention on Human Rights to "manifest her religious belief".
Mr Diamond told the appeal judges - Lord Justice Maurice Kay, Lord Justice Elias and Lord Justice Vos: "It is our case that this appeal raises a number of important issues".
Merton council lawyers are arguing that the employment tribunal which first dismissed Mrs Mba's claim was "plainly and unarguably right" and the EAT was justified in upholding its decision.
Mrs Mba, who is now a childcare worker with a company which allows her to have Sundays off, stresses that she is not attempting to force her beliefs on anyone else, but argues an employer has a duty to "reasonably accommodate" the beliefs of a Christian employee.
If she wins her appeal, it could establish new religious rights allowing Christian workers generally to avoid Sunday working.
Her lawyers are relying in part on a European Court of Human Rights (ECHR) judgment issued in January over a case brought by four Christians on religious rights in the workplace.
The court ruled that Coptic Christian Nadia Eweida, a British Airways worker who was sent home from work for displaying a small silver crucifix during her job as a check-in attendant at Heathrow Airport, had been discriminated against under freedom of religion laws.
But it rejected similar claims brought by the three other Christians.
Andrea Williams, of the Christian Legal Centre, which is backing Ms Mba, said: "We are finding that when it comes to Christians, they are not accommodated in the way that other faiths are accommodated.
"We are seeing secular courts ruling on core components of Christian practice.
"The courts have acted to protect the kara bracelet (worn by Sikhs), Afro cornrow haircuts, the wearing of the hijab and a Muslim's right to fast, but have refused to grant the protection to the cross or the Christian Sunday."
Shop workers are already entitled to refuse to work on Sundays, but employees in other areas can be required to do so if their employer shows there is "a legitimate business need".
The judges reserved their decision and said they would hand down their judgment in writing at a later date.