Edexcel, AQA and OCR are the biggest exam boards in the country. The government's qualification regulator, Ofqual, has sent a letter to all exam boards asking them to ensure their papers are error free. Glenys Stacey, Ofqual's chief executive, said: "The recent run of exam errors are disappointing and unacceptable. While the vast majority of question papers taken so far have been free from error, there have been a number of question papers that have included errors. We take instances like this very seriously. I am calling on awarding organisations to take steps now to protect students from further disruption and anxiety."