The qualifications watchdog has warned it could stop exam boards from operating, as it investigates the latest in a "unique" string of errors, condemned as unacceptable by the prime minister this week.
A total of 11 cases from this summer's GCSEs, A- and AS-levels are being looked into by Ofqual. Pupils could sue over the errors, which they fear could jeopardise university places, it was reported this week.
They involve all three of England's main exam boards and consist of a printing mistake, two papers widely alleged to be unfair and eight actual errors in questions, rendering some of them unanswerable.
The latest question errors came in two OCR exams - a GCSE Latin paper and an A-level physics paper - sent out after Ofqual warned boards to carry out new checks on all their papers earlier this month.
The watchdog described the mistakes as "unacceptable" and said they brought into question the assurances given by exam boards.
Glenys Stacey, Ofqual chief executive, told The TES that she had sent her own team into OCR this week to check on their papers to ensure there would be no more mistakes.
"(We) are just checking through, given their recent errors, what they have done to really put into place the assurances they have given us and what they are doing to ensure the 100 or so papers that are still to be sat (are error-free)," she said.
She added that late changes to exam papers and human errors seemed to be the factors behind this summer's mistakes. Ofqual will start a full inquiry next week.
"We have a range of powers," Ms Stacey said, when asked what sanctions it could use against exam boards. "Ultimately we could actually stop an awarding body from practising."
But she said it was not at that stage yet because the main priority was to prevent further errors and ensure pupils who had taken the papers in question were not unfairly disadvantaged.
Official records show that no such exam errors were reported in 2009 and 2010. But Ms Stacey said comparisons with the 11 cases uncovered this year may not be accurate because the rules on exam boards reporting mistakes had changed.
She said that media attention and pupils communicating through social media websites such as Facebook may also have helped explain why numbers were so high this year.
But Ms Stacey added: "This is a very unusual situation, if not unique."
In Parliament this week, prime minister David Cameron said: "This is not an acceptable situation.
"I've discussed it with the education secretary (Michael Gove) this morning who in turn has discussed it with Ofqual, who are taking the toughest possible action to root out this failure and make sure it doesn't happen again."
Mr Gove is "angry" about the errors and believes the exam system is "discredited".
An OCR spokesman said: "We deeply regret these errors. We are extremely angry, because this is not fair on students, parents and teachers.
"It is not acceptable, and if we find that someone has not done their job, they will lose their job."
Privately, exam boards are unhappy at the public rebukes issued by Ofqual over the mistakes, arguing that it was wrong to issue them during the exam season.
Ms Stacey said: "The notion that it should or could be kept out of the public domain might be a little naive given the media world we live in and the sort of social-media activity that concerned students will participate in."
Live exam paper errors reported to the regulator:
2006: One incident
2007: One incident
2008: Three incidents
2009: No incidents
2010: No incidents
2011: 11 incidents being investigated so far
Ofqual says reporting rules have changed and it cannot be certain that the above figures include every incident in the past. They should be "treated with caution" and not used for direct comparisons with 2011.