Lillian Ladele, the Islington registrar who refused to perform civil partnership ceremonies for gay couples, will argue her case in the court of appeal today.
Ladele, 48, from Finsbury, was threatened with the sack after she told Islington borough council in 2007 that she could not carry out the ceremonies due to her Christian beliefs.
An employment tribunal in 2008 had ruled in her favour but the council successfully appealed the decision at the Employment Appeals Tribunal (EAT).
She is appealing against the EAT’s ruling today.
Her lawyers will argue she was the victim of a witch-hunt and had been shunned by her colleagues.
Mike Judge, of the Christian Institute, which is assisting Ladele, said: “Islington council accepts it was able to provide civil partnerships registrations without forcing Lillian to take part.
“They could therefore have reasonably accommodated Miss Ladele’s genuinely held religious belief without affecting service delivery.
“That would have been a balanced approach which respected both sides of the debate. Instead they chose to make gay rights more important than religious rights.
“If this decision is allowed to stand it will help squeeze out Christians from the public sphere because of their religious beliefs on ethical issues.”
Human rights campaigner Peter Tatchell told the Guardian: “This case is part of a homophobic fight-back by Christian fundamentalists who resent the removal in law of their right to discriminate against lesbian and gay people.
“Faith can never be a legitimate reason to seek exemption from the law and the moral obligation to treat other people with respect and equality.
“The issue is very simple. Gay people have no right to discriminate against religious people, and religious people have no right to discriminate against gay people.”
The hearing is expected to last for two days.