The funding of religious education (RE) in UK state schools is "frighteningly dependent" on the head teacher's personal stance, research suggests.
James Conroy, professor of RE at Glasgow University, says some schools spend just £1 per pupil a year on RE.
His report found RE was becoming less about faith issues and could cover anything from sex to citizenship.
The Department for Education said it was "down to schools themselves to judge how it is taught".
Prof Conroy said the subject was under-resourced, overlooked and overburdened.
His research is based observation and interviews at 24 UK schools, some faith-based, others not, some in rural settings and others in urban areas.
It will be presented at the second of the Westminster Faith Debates, which focus on faith in schools, on Wednesday.
Prominent atheist Richard Dawkins; John Pritchard, Bishop of Oxford and chair of the Church of England's board of education; and Professor Robert Jackson, director of the Warwick Religions and Education Research Unit are also taking part in the debate.
Prof Conroy said: "RE in Britain is under-resourced, torn between competing aims, and has become overburdened by having to include other subjects (from sex to citizenship).
We cannot understand our own culture without religious knowledge, let alone that of others”
End Quote James Conroy Professor of religious and philosophical education at Glasgow University
"Whilst governments insist on RE's importance in theory, they marginalise it in practice - as Michael Gove has recently done by refusing to treat it as a core subject and excluding it from the English Baccalaureate."
The subjects in the English Bacc award are English, maths, science, a modern foreign language and a humanities subject - either geography or history.
Prof Conroy's study said that in most state schools, £1 or less was spent on each child per annum on materials and books for RE.
The report said: "Religious education matters as never before. We cannot understand our own culture without religious knowledge, let alone that of others.
"As religious and secular diversity increases, students need to be able to articulate their own beliefs, and engage seriously with those of others, as never before.
"What is happening to RE in our schools is a scandal for which we will have to pay a high price in years to come."
The research comes after a group of cross-party MPs pledged to come together to safeguard the teaching of RE in England.
Once established, the group will work to establish the value of the subject.
A Department for Education spokesman said: "RE remains a statutory part of the school curriculum for every student up to 16.
"It is rightly down to schools themselves to judge how it is taught, but the English Baccalaureate will not prevent any school from offering RE GCSEs.
We have been clear that pupils should take the GCSEs that are right for them and that we look to teachers and parents to help pupils make the right choice."