Humanists are preparing to mount a legal challenge against a Government decision to exclude "non-religious" views from the subject content of the latest religious studies GCSEs.
Three humanist families are scheduled to mount a challenge in the High Court in London on Tuesday - with the backing of the British Humanist Association (BHA).
Officials at the BHA say the content upon on which examination boards must base specifications fails to allow for the in-depth study of "a non-religious worldview" - such as humanism.
BHA chief executive Andrew Copson said: "The law is clear that when teaching about religions and beliefs, schools should follow a broad and balanced syllabus which includes both religious and non-religious worldviews like humanism on an equal footing.
"The religious studies GCSE is therefore incompatible with the rights of non-religious parents and the entitlement of young people to an education that does not indoctrinate them, even by omission.
He added: "It amounts to a breach of the Government's obligations."
A BHA spokesman said three sets of humanist parents who had children approaching GCSEs were bringing legal action - and asking for a "judicial review".
He said the families came from Cumbria, Somerset and Kent.
Parent Kate Bielby, of Frome, Somerset, said: "I completely recognise the importance of children learning about the different religions, especially in our increasingly diverse society.
"What I object to is the lack of parity between religious beliefs and non-religious worldviews in the school curriculum, which in the eyes of children may well lead to the belief that religion, in whatever form, has a monopoly on truth and on morality."
She added: "This is not accurate, it reflects neither the views of the population nor the traditions of the country, and we shouldn't be encouraging our children to believe it."
The BHA spokesman said the Government decision - made in November 2014 - to exclude humanism was "condemned" by philosophers, academics, teachers and religious leaders including former Archbishop of Canterbury Rowan Williams.