Thousands of teenagers were asked to answer an 'impossible question' during a maths AS-level exam set by the OCR exam body last week.
According to the BBC, the test was sat by 335 schools and exam centres in England, Wales and Northern Ireland.
The question was worth eight marks in total.
One exam candidate told the BBC that the question "threw" him and caused stress.
"Many people are worried that the mistake made by the examining board will severely affect the mark and grade they achieve in the paper. For many this was a final exam and will most likely influence final grades and university admission," he told the news provider.
A spokeswoman for OCR said the organisation regrets the mistake and will ensure that students are not disadvantaged by the error.
She added that letters have been sent to schools and colleges explaining what action will be taken.
Posted by Katy Kearns