The government will be challenged in court tomorrow over its decision to exclude non-religious beliefs from the new Religious Studies GCSE syllabus.

Under the new curriculum, which will be taught in schools from next September, pupils will be required to study two religions but not humanism. Three parents are taking the government to court arguing that a non-religious worldview should have equal footing with a religious one.

"I completely recognise the importance of children learning about the different religions, especially in our increasingly diverse society," said Kate Bielby, one of the three parents leading the action.

"What I object to is the lack of parity between religious beliefs and non-religious world views 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.

"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," she added.

The judicial review case, which is expected to last one day, is being supported by the British Humanist Association (BHA).

"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," said BHA's chief executive, Andrew Copson.

"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.

"It amounts to a breach of the government's obligations and that is what we are seeking the court to affirm, in line with overwhelming public and professional opinion."

After the government announced the policy, 28 religious leaders urged a re-think. The former Archbishop of Canterbury Rowan Williams was among the leaders who encouraged the government to allow students "the option of systematic study of humanism in GCSE."

The plans were launched over a year ago by the Department for Education to develop a "more academically rigorous qualification." The in-depth study of two faiths will make up half of the new GCSE. The rest of the course is based on the study of philosophy and ethics which the government says can include humanism and other non-religious beliefs.

A ruling on the challenge is expected within a few months.