Examination boards are abandoning an attempt to recruit clergy to mark GCSEs in religious studies because their knowledge of world religion is not broad enough.
Modern exam papers have so many questions about Islam, Sikhism and Hinduism that many Christian clerics are not qualified to mark them properly.
The boards, which have to grade 300,000 GCSE entries each year, wrote to the Church of England and the Roman Catholic Church, asking for vicars and priests to work as markers because of a shortage of examiners.
Most of the clergy contacted have declined, however. Many said they were unable to mark papers that often contained more questions on other world religions than on Christianity.
Paul Sokoloff, the head of qualifications at Edexcel, one of the country's three main exam boards, confirmed that a lack of knowledge about other faiths seemed to have deterred the clergy.
"There was no sudden rush of vicars coming forward to examine," he said. "The span of knowledge required - which covers all the major religions - and the level of detail needed to mark the GCSE was a factor."
Canon John Hall, the general secretary of the Church of England's board of education, blamed the lack of Christianity in modern GCSEs for the failure of his colleagues to volunteer for exam marking. "Not all clergy would be qualified to examine unless they were teachers," he said.
Pupils have a choice of seven GCSE courses in religious studies. Only one is based exclusively on Christianity. In the other six, pupils choose between modules on Christianity, Islam, Hinduism, Buddhism, Sikhism and Judaism.
In three of the syllabuses it is possible to avoid Christian study altogether. In the Islam option in one GCSE syllabus set by Edexcel, pupils are asked to answer questions about the Koran's instruction on the relationship between the sexes and Muslim attitudes to homosexuality.
In a syllabus set by another exam board, the OCR, pupils learn about the Hindu gods Brahma, Vishnu, Shiva and Krishna, and concepts such as karma.
The Rev Gill Marshall, of Freston, Ipswich, said the presence of such topics would make it impossible for her to mark GCSEs. Her training had concentrated on Christianity.
"Although we visited the odd mosque when I was training, we didn't do other religions in any depth," she said. "I would not presume to consider signing up to be an examiner. I think it's something teachers should do."
The Rev Derek Marr, of West Denton, Newcastle, said: "I'm nearly 70, so it's not for me, but there are a lot of bright, up-and-coming clergy who might have the knowledge to take it on."
Lat Blaycock, a spokesman for the Professional Council for Religious Education, which represents RE teachers, said clergy were the wrong people to mark exams. This should be done by teachers.
He said: "Vicars are no more qualified to mark GCSE exams in RE than people who work in Boots are qualified to mark chemistry papers."