The number of Faith schools has grown in Britain
The government is facing criticism over claims that it allows faith schools to refuse jobs or promotion to staff with different, or no, spiritual beliefs.
The National Secular Society is arguing at its conference that current legislation discriminates against well qualified, non-religious teachers,
The Department for Children, Schools and Families has said it has not received any complaints.
It said beliefs are only taken into account in a small number of cases.
Currently, the School Standards and Framework Act 1998 allows faith schools in England and Wales to take into account an applicant's religious beliefs, or lack of them, when considering them for a teaching post.
This is intended to ensure that staff fully support the school's particular religious ethos, such as Christianity or Judaism.
The Act was later amended to include school support staff.
In Scotland, Catholic faith schools are allowed to request that applicants provide a suitable referee who can testify to their personal religious beliefs and character.
Catholic teachers are asked to provide a testament from their parish priest.
The NSS has told the BBC that such requirements are discriminatory, and have no bearing on whether a person can teach a particular subject well.
The Executive Director of the NSS, Keith Porteous Wood, will tell the Windsor Castle conference that the legislation is "appalling".
"As religious observance plummets, it is unfair for non-religious teachers to be discriminated against in employment.
"How religious do you have to be to teach maths?"
However, Mr Porteous Wood will argue that teachers can accept a school's religious ethos, and even support it to the extent of attending acts of worship with the pupils, as long as they are not required to actually believe it.
Some critics are worried about how far a faith school teacher might be required to reflect the school ethos, and whether their private lives could come under scrutiny.
Earlier this year a headteacher at a Catholic primary school in Sheffield resigned over his plans to remarry a second time.
Michael Cassidy told parents in a letter that "... my position as head teacher is no longer compatible with the conditions and services of my contract. Therefore I have no option but to leave."
Should teachers have to uphold their school's religious beliefs?
The Catholic Church teaches that marriage can never be completely dissolved and therefore marrying a second time is adultery in the eyes of God.
In a letter to the pressure group Accord in October, schools minister Vernon Coaker admitted that school governors did have the power to decide if a staff member's conduct was damaging to a school's ethos and were entitled to take appropriate action.
But a spokesperson for the Department for Children, Schools and Families told the BBC that any school wishing to discriminate on the grounds of faith had to demonstrate, if challenged, that there was a 'genuine occupational need'.
"No one has come to us complaining of discrimination."