Posts by sixth formers in an online forum also suggest some knew in advance what the subject was going to be.
The exam board, OCR, is investigating and has apologised for any distress.
The exams watchdog the QCA and Mr Cairns have called on the board to provide a full explanation.
A spokesperson for the QCA said it would make sure that any remedial action taken was fair to all candidates.
It was not immediately clear how many candidates in how many schools and colleges were affected.
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