Religious leaders have urged the government to include Humanism in the new religious studies GCSE.

Former Archbishop of Canterbury Lord Rowan Williams is among 28 faith leaders who signed a letter to Schools Minister Nick Gibb, calling on him to add Humanism to a list of faiths included in the proposals.

Under the current plan, students will have to study two major faiths out of Christianity, Catholicism, Judaism, Islam, Hinduism, Sikhism and Buddhism. It is hoped that this will increase religious literacy among the younger generations. Critics, however, believe that pupils should have the option to study a non-religious or humanist worldview.

"As religious leaders we are writing to express our support for proposals to allow students to have the option for systematic study of Humanism...and for an annex setting out content on Humanism to be added alongside existing GCSE annexes on the principal world religions," the letter reads.

"Such a change would not compel anyone to systematically study non-religious worldviews 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 worldviews that are common in Britain today."

Signed by representatives including the Rt Rev Richard Harries, former Bishop of Oxford, Rabbi Dan Cohn-Sherbok, a Professor of Judaism, and Raheed Salam of the Interfaith Youth Trust, the letter adds that including Humanism "would be fair, popular, and add rigour to the subject".

"We see no reasonable or persuasive argument to oppose it," it concludes.

The call has been welcomed by the British Humanist Society (BHA), which was asked by the Department of Education to produce the content of an annex on Humanism.

The group has branded the later decision not to include it "inexplicable".

In a statement released today, Chief Executive of the BHA, Andrew Copson, commended the letter, saying it was "great to see religious leaders of good will" fight for the cause.

"A Government decision not to include the annex will 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," he added.

"It is completely inexplicable to us why the Government would take this arbitrary and counter-educational decision in the face of public opinion and professional support."

The BHA has previously argued that including Humanism in GCSE religious studies courses is vital because pupils must learn "that there are many who do not believe or practise a theistic or religious worldview".

"Indeed if pupils did not learn this, it could be said they were victims of indoctrination," the group has said. "It is vital that RE stays relevant to our population if it is to maintain its place within the curriculum."

Dr Farid Panjwani, Director of the Centre for Research and Evaluation in Muslim Education at the Institute of Education, also offered his full support to the campaign.

"Such an inclusion would contribute to promoting intellectual autonomy among students by giving them opportunity to learn about a broader range of ways in which humans have grappled with existential questions," he said.

Teaching of the new GCSE qualification will begin next year. According to the BBC, the Department of Education says it worked with "all the major faith groups" to plan the syllabus, and it will help students to form a more rounded view of beliefs in modern society.

A Department for Education spokeswoman said that students who wanted to study Humanism at GCSE level would have the option 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 per cent of the course studying philosophy and ethics; which can include studying humanism and other non-religious beliefs," she said.