A series of at least nine errors in this year's GCSE, A-level and AS-level exam papers is "not an acceptable situation", Prime Minister David Cameron has said.
He said England's exams watchdog Ofqual would take the "toughest possible action" to stop further errors.
Ofqual said it was meeting with exam board heads and would launch an inquiry this summer.
Three more mistakes emerged on Tuesday, in AQA and OCR exam papers.
Both exam boards apologised for the mistakes, which affected OCR's A-level physics and GCSE Latin papers, and AQA's GCSE maths foundation paper.
Speaking at prime minister's questions, Mr Cameron said he had discussed the issue with Education Secretary Michael Gove, who had spoken to Ofqual.
The qualifications regulator was "taking the toughest possible action to root out this failure and make sure it doesn't happen again", he said.
In a statement on Wednesday morning, Ofqual said it was "unacceptable" that candidates had faced fresh errors after exam boards had provided written promises that additional quality checks would take place after the first wave of errors.
"Our concerns are serious enough that our director of regulation is meeting with the chief executive of OCR today to understand how these recent errors occurred and to make sure, so far as possible, that there will be no more avoidable errors," the statement said.
Regulators would be meeting on Thursday with heads of other exam boards and would "again ask for confirmation that there will be no more avoidable errors", Ofqual said.
When the exams are over, full details of an inquiry will be announced, it said.
"We want to understand how this pattern of errors occurred, how errors remained after additional checks had been made and what needs to be done to prevent any recurrence of such avoidable errors."
On Tuesday, a spokesman for the Department for Education said the latest errors were "completely unacceptable" and that Mr Gove was "angry".
"He has said repeatedly that the exam system is discredited and action must be taken," the spokesman said.
In the Latin GCSE error, a paper taken by up to 8,000 pupils contained three errors, representing 14 out of 50 marks.
An error in a question in the A-level physics paper, taken on Tuesday morning, meant that pupils were asked to answer a question which included incorrect units of measurement.
And a printing error on a maths GCSE foundation paper, also taken by pupils on Tuesday morning, had questions from a previous paper accidentally printed in the middle of it.
An OCR spokesman said the exam board was "deeply unhappy" about the latest errors. It said there would be an investigation, and jobs could be lost as a result.
AQA said it was sorry that the error had caused some students distress. It said the batches of papers that were checked were all fine, and it was investigating with its printer how the problem had arisen.
Six other errors in previous papers taken this year have included a multiple-choice question where all the answers were incorrect, and a question where the information needed to make a calculation was missing from the paper.
These appeared in papers produced by the Edexcel exam board, and the Northern Ireland CCEA board, as well as AQA and OCR, and have affected exams taken in England, Wales and Northern Ireland.