Teachers and pupils have been left angry after exam board OCR made their third mistake in just over a fortnight.
The exam board has apologised for not providing a vital formula in the A-level Biology paper after their own guidance to schools said it would be included.
In yesterday's exam, pupils were asked to calculate standard deviation – but the formula was nowhere to be seen.
A petition was launched last night calling on OCR to "cancel" the paper or lower the grade boundaries. More than 4,000 people have already signed the petition, which describes the exam as "one of the hardest papers" ever seen.
Teachers, parents and pupils also took to Twitter to complain about the error:
Just last week, OCR apologised for "poorly wording" a question in an A-level psychology exam sat on Wednesday.
It came after the exam board had to apologise for an error in a GCSE English Literature exam question on Shakespeare's Romeo and Juliet at the end of May.
OCR has said it will consider the errors in all three exams in the marking and grading process this summer.
A spokesperson for OCR told Tes: “For one question on OCR’s A-Level Biology paper H420/01 (Question 20a, worth 3 marks out of a total of 100), the formula for standard deviation was not provided.
"Some of OCR’s guidance to schools and teachers about the specification indicated that candidates would be provided with this. OCR apologises and will address any impact of this during marking and grading."