The former Archbishop of Canterbury Rowan Williams has urged the government to rethink plans to leave humanism out of the new GCSE in religious studies.
Lord Williams is among 28 religious leaders who have written to Schools Minister Nick Gibb arguing for the study of humanism to be included.
The new qualification is due for first teaching in England from 2016.
The government says students can choose options "which can include humanism and other non-religious beliefs".
The Department for Education says it worked closely with experts from "all the major faith groups" to develop the new "more academically rigorous". qualification which would give students better knowledge and understanding of the diversity of beliefs in modern Britain.
The proposed course will require pupils to study two faiths and aims to develop a stronger understanding of the central role of religion on British culture.
The faiths included in the proposals are Buddhism, Christianity, Catholic Christianity, Islam, Hinduism, Judaism and Sikhism.
The plans have been broadly welcomed by leaders of major faith groups but there is concern that there is no option to study a humanist or non-religious world view.
The letter says students should "have the option for systematic study of humanism in GCSE, AS and A-level religious studies."
The authors want "an annex setting out content on humanism to be added alongside existing GCSE annexes on the principal world religions".
"Such a change would not compel anyone to systematically study a non-religious worldview or make it possible to do so for the whole of a qualification, but it would allow young people to study a more representative sample of major world views that are common in Britain today."
The RE Council for England and Wales and the National Association of Teachers of RE already support such an approach, argues the letter.
"In short, it would be fair, popular, and add rigour to the subject; we see no reasonable or persuasive argument to oppose it."
The letter was also signed by the former Bishop of Oxford, Lord Harries, as well as academics, teachers and Jewish and Sikh religious leaders.
British Humanist Association chief executive Andrew Copson welcomed the letter, arguing that leaving out humanism would "make a joke of the idea that religious studies will be accessible to every child, undermines the contemporary relevance of the qualification, and reduces the meaningfulness and rigour of the subject".
Dr Farid Panjwani, director of the Centre for Research and Evaluation in Muslim Education at the Institute of Education said including humanism would allow students "to learn about a broader range of ways in which humans have grappled with existential questions".
An RE teacher who supports the campaign said he feared the government's current proposals would alienate many pupils and "prevent us from facilitating proper understanding and from having time to discuss and explore issues that are fundamental to our existence".
A Department for Education spokeswoman said GCSE students who wanted to study humanism would be able to do so.
"The proposed new GCSE requires students to have an understanding of the beliefs, teachings and practices of two religions but still allows them to spend up to 50% of the course studying philosophy and ethics; which can include studying humanism and other non-religious beliefs."