Many children may be missing out on religious education at school, campaigners have suggested.
A new report claims that up to one in four secondaries in England are struggling to meet their legal obligation to teach pupils about major religions and belief systems.
A survey conducted as part of the paper, published by the Religious Education Council and the National Association of Teachers of RE (NATRE), found that a quarter of the schools polled said they do not offer the subject to all students at GCSE level (aged 14 to 16).
This is likely to be fuelled by a range of factors, the report suggests, such as the fact that RE is not included in the English Baccalaureate - a school performance measure that recognises youngsters who studied a group of academic subjects.
In addition, the survey found differences between types of schools - with 96% of faith schools saying they offer the subject to all 14 to 16-year-olds, compared with 73% of academies.
Information gathered by the two organisations from the Government's workforce census under Freedom of Information requests also suggests that some schools are not teaching RE.
The request asked for the number of hours of RE each secondary school in England taught to each year group - from Year 7 to Year 11.
For each year group, the proportion of schools teaching no hours in 2015 was around one in four, the report calculates, with the highest proportion being around 28% for Year 11.
NATRE's research officer, Deborah Weston, said: "Whilst many schools, including academies and free schools, are continuing to deliver good RE, these statistics highlight serious problems that have implications for cohesion and inclusivity in our society, as well as presenting questions around the role of specialist RE teachers in schools.
"By developing knowledge and understanding about different religions and world views in the security of a classroom, young people have the opportunity to engage with complex, diverse and constantly evolving subject matter."
But Geoff Barton, general secretary of the Association of School and College Leaders, said schools may be teaching the subject in different ways, rather than through specific RE lessons.
"They might be teaching through conferences, they might be using citizenship lessons, they might be using assemblies," he told the BBC.
A Department for Education spokesman said: "The Government firmly believes in the importance of religious education. Good quality RE can develop children's knowledge of the values and traditions of Britain and other countries, and foster understanding among different faiths and cultures.
"Religious education remains compulsory for all state-funded schools, including academies and free schools, at all key stages and we expect all schools to fulfil their statutory duties."
It is up to schools to decide how to offer RE, whether it is through classes in the subject, or alongside other topics, the DfE said.