A quarter of state secondary schools in England are breaking the law by not teaching religious studies to teenagers, a poll has found.

Under the law, state schools must teach the subject to under-16s, regardless of whether they are studying religion for a GCSE. The law does not specify for how long the subject should be taught.

The National Association of Teachers of Religious Education (Natre) warn that the schools are starting to "kill off" the subject.

A quarter of 1,517 state RE teachers polled by the association said their schools were neglecting to fulfil their statutory duty to teach the subject. This rose to 31% among grammar school teachers.

The teachers, however, said the English Baccalaureate was to blame. The Ebacc is the government's new way of measuring schools in league tables. A pupil is counted as achieving the Ebacc if they obtain A*-C grades in five GCSE subjects: English; maths; two sciences; ancient or modern history or geography; and a modern or ancient language.

Nick Gibb, the schools minister, has said Ebacc subjects equip students for the "most competitive courses". By encouraging schools to enter pupils for these subjects, more opportunities will be available to them, he has argued.

But Ed Pawson, chair of Natre, said excluding RE from the Ebacc "effectively squeezed it out of the curriculum".

Teachers polled said pupils' interest in studying the subject to GCSE had fallen by more than a third.

One teacher said their school planned to stop offering a GCSE in RE "to focus on EBacc subjects and English literature".

A spokesman from the Department for Education said the list of subjects in the Ebacc had been kept deliberately small to allow the opportunity for wider study.

"There are valuable and rigorous qualifications not in the EBacc, like RE, which pupils should be free to take if they want," he said. "RE remains a vital part of the school curriculum – that's why it remains compulsory for every student up to 16. It's down to schools to judge how it is taught and how it fits into wider school life."