Almost one in three secondary schools is planning to slash the time devoted to teaching RE from September following the introduction of the English Baccalaureate, according to a survey of almost 800 schools.
The radical curriculum overhaul has been caused by concerns that schools will plummet in league tables under the controversial new measure, the National Association of Teachers of Religious Education (NATRE) said.
To qualify for the EBac pupils must achieve at least a grade C in GCSEs or IGCSEs in English, maths, two sciences, a foreign language and a humanity. RE has not been included on the humanities list, prompting outrage from teachers and religious leaders and fears that the subject will be marginalised.
The NATRE survey - which gathered almost 800 responses from 4,200 state and independent schools - found planned cuts to both short and full-course GCSEs in religious studies from this September. In some cases schools are reported to be ignoring their statutory duty to offer RE at all.
NATRE executive officer Rosemary Rivett said: "We are very concerned at the speed with which schools have reacted to the EBac and their willingness to put their statutory requirements to one side.
"Schools appear to be feeling that it's quite safe to cut RE without any fear that they will be held to account.
"It's not just options and courses, but teaching staff that are being affected. If schools are making teachers redundant or not appointing them, they will struggle to offer courses in future should they or the Government change their minds."
Officials from the Department for Education have said they are concerned about the survey's findings and have asked for a full break-down of its results, Ms Rivett said.
The poll follows a smaller-scale survey released last week by the National Association of Music Educators, which reported that 57 out of 95 schools were planning to cut opportunities to study music from September.
It also came as Stephen Lloyd, Liberal Democrat MP for Eastbourne and Willingdon, this week tabled an early-day motion calling on the Government to recognise RE in the EBac.
"The rise of religious extremism around the world and in the UK means that a good understanding of all religions is vital to a well rounded education," he said. "I am concerned that the Government is not currently recognising this by failing to include the subject in the new qualification."
Church of England chief education officer Reverend Jan Ainsworth said: "We are astounded by the omission of RE from the list of subjects that count for the Baccalaureate. RE is just as academic and rigorous as history and geography.
"We call on the Government to rethink this policy and put RE back where it belongs."
Education secretary Michael Gove said last month that he would "take on board" constructive comments about the qualifications that count towards the EBac, but refused to promise that any changes would be made.
A spokesman for the DfE added: "It will be up to schools to decide on their offer but it remains compulsory for schools to teach RE and success in the GCSE will continue to be recognised by other performance table measures."
GEOGRAPHY - Beset by 'uninspiring teaching'
Geography - one of the pillars of the new English Baccalaureate - is a subject in decline, with student numbers and teaching standards on the wane, according to Ofsted.
The subject is afflicted by "uninspiring teaching" and insufficient challenges in secondary schools, according to a report released by the inspectorate today.
Pupils' knowledge of places is described as "exceptionally weak", with more than half of schools failing to use fieldwork to "nurture a love of geography".
New figures reveal 90,000 fewer students took GCSE geography in 2010 than in 1995 - a drop of more than one-third.
Ofsted's report concluded that a "focus on factual recall rather than on exploring ideas failed to capture students' interest".
The subject is also "more or less disappearing" in one in 10 of the primary schools visited by inspectors.