An exam board has apologised "wholeheartedly" after making a mistake in an English literature GCSE paper on one of Shakespeare’s most famous plays.

Teenagers sitting the exam, set by OCR, on Friday were left confused when a question on Romeo and Juliet suggested key character Tybalt was a member of the Montague family when he is a Capulet.

In a statement, OCR apologised for the mistake.

It said: “To ensure all students are treated fairly, we will put things right when the exam is marked and graded.

"To do this, we combine feedback from our examiners with statistics to measure the potential impact on students.

"We are then able to make appropriate adjustments – including to the grade boundaries if necessary – to ensure no-one is disadvantaged.

“Once again, we are sorry for this error on one of our exam papers and we are investigating how it got through our assurance processes as a matter of urgency. We are liaising directly with schools, and advise students and families to talk to their teachers for further information.”

Thousands of students across the country could be affected by the error, including two thirds of IPACA’s GCSE students.

Acting principal Gary Spracklen said it was a shock to see the mistake.

“The error affected about two thirds of our GCSE students as the other students studied Macbeth,” he said.

“Our students are in the same situation as all the others around the country who took the exam. It was just a shock really. The students had to learn 15 poems, two plays and one novel; all we ask is for the exam board to get the question right.

“It threw them a little bit. Some recognised the error straight away and as the question was multiple choice, they did not have to answer that one. It puts them at a disadvantage because it means they didn’t have a broad choice. It’s hard to see how it won’t affect them. After five years of work it is frustrating.”

Exam board regulator Ofqual says the mistake is “unacceptable” and insists it will now closely monitor OCR’s investigation in to how the error occurred.

In a statement, the regulator said: “We are very disappointed to learn of the error in OCR’s English literature exam paper.

"Incidents of this nature are unacceptable and we understand the frustration and concern of the students who may have been affected.

“We will be scrutinising how OCR intends to identify and minimise the impact on these students. We will be closely monitoring OCR’s investigation of how this incident occurred and seeking reassurance regarding its other papers this summer.”

Geoff Barton, general secretary of the Association of School and College Leaders, is calling on OCR "to take appropriate action to make sure that candidates are not in any way disadvantaged.”

Mr Barton said: “Candidates have every right to expect that awarding bodies complete a full check on exam papers to ensure that they don’t experience such problems.

"Similarly, schools and colleges have to pay thousands of pounds a year to examination boards and are entitled to better quality assurance than this. This appears to be a serious error and it will have caused stress and concern to candidates.

"Students need to be able to perform to the best of their ability and seeing errors in a paper can undermine their confidence."