Jewish students could be "disadvantaged" by a new requirement to show knowledge of Christianity for GCSE religious studies, the Department for Education has been told.
They will have to demonstrate it even if they study Islam as a second religion after Judaism.
The government last year said that from this September, pupils would have to devote at least a quarter of their GCSE studies course to a second religion. Up till now they have been able to focus exclusively on one faith.
Chief Rabbi Ephraim Mirvis recommended that Orthodox schools should teach Islam as a second religion.
But now it has emerged that students will also be expected to demonstrate awareness that Britain's religious traditions are principally Christian.
Education agency Partnerships for Jewish Schools (Pajes) said that it had raised with the DfE "our concern that the current format of the proposed religious studies GCSE significantly disadvantages students who do not select Christianity as one of their chosen religions".
One Jewish studies teacher said that this would be "discriminatory" to Jewish pupils as they would effectively have to learn about a third faith - whereas other students taking Christianity as one of their main options would need only to study a second faith.
Michelle Webb, religious studies specialist with OCR, one of the three English exam boards, said that course content reflected "strict criteria" set out by the DfE.
A Board of Deputies representative said it was "comfortable with the content". A spokesman for Rabbi Mirvis said: "It is disappointing that the structure which the main exam boards have chosen… will present significant challenges for Jewish and indeed many other faith schools."