Teachers' leaders are calling for a review of the GCSE and A-level exam schedule next year to help Muslim students cope with a clash between it and Ramadan.
Students who will have to fast during the exam period are bound to lose marks as a result, the Association of Teachers and Lecturers' annual conference in Manchester was told today.
Discussions are being held between the ATL, the Joint Council for Qualifications and leading Muslim groups to see if any can be offered to the students - possibly by ensuring that students do not have to sit too many exams on the same day.
Barry Lingard, of the ATL executive and from Bolton, said: "For up to half a million students, Muslim students, we can see a very large problem looming. The observance of Ramadan, the ninth month of the Islamic calendar entails fasting from sunrise to sunset."
Ramadan moves 10 days forward every year - which means it will clash with exams for the next five years. The last time this happened was 25 years ago when there were far fewer Muslims in the UK.
Mr Lingard said it was "highly unlikely" exam boards would change timetables, although the JCQ had agreed to review whether a balance of morning and afternoon slots for large entry exams could be arranged for morning slots only.
"This issue is even more important with the proposed move towards linear assessment [more reliance on the end-of-course exam]," he added. "Young Muslim pupils will have one shot at taking these examinations. And over the coming years young Muslim pupils will be at, what we believe, is a severe disadvantage."
Abdul Choudhury, from Tower Hamlets, added: "I have no doubt whatsoever that standards will be affected by this. We work so hard to get them maybe that extra mark or two but if they go into the exam without any water or any food it will have an effect on their results."
Michael Turner, director of the JCQ, said: "We're consulting on the exams timetable for 2015 and we will be looking at the impact of Ramadan on that timetable."
Next year Ramadan starts on 18 June, the following year 7 June, and in 2017, 27 May.