England's exams regulator Ofqual has told the Education Secretary Michael Gove it has concerns over some of the changes he is bringing in to England's exams system.
GCSEs are being overhauled in England and from 2015 some are being replaced with new English Baccalaureate Certificates.
The watchdog has written to Mr Gove setting out its concerns.
A letter it wrote to him was released this lunchtime.
The release follows tense exchanges between Mr Gove and MPs in the Education Select Committee this morning.
He confirmed he had received a letter from Ofqual, but refused repeated requests from the MPs to reveal what was in it.
Mr Gove said it was up to Ofqual if it wanted to release the letter, but that he would be meeting officials next week to discuss the concerns.
Committee chairman and Conservative MP Graham Stuart said it was "unacceptable" that Mr Gove would not discuss the concerns with MPs.
That letter has now been released by Ofqual.
In it, the regulators say they support the aims of the changes being brought in, but question whether the new qualification (English Baccalaureate Certificate - EBC) will be able to achieve all of what is being expected of it.
"Our first concern is that the aims of the EBC "may exceed what is realistically achievable through a single assessment", its letter says.
The watchdog appears to question whether it is possible to have an exam which can provide reliable data for league tables and which at the same time is capable of being taken and passed by teenagers of all abilities.
It also says the EBC, with its planned focus on depth of knowledge and longer essays, will not be a reliable indicator for league tables, because of the subjectivity involved in marking such exams.
And it calls for a delay in the move towards the appointment of one exam board per EBC subject.
At the moment, exam boards offer different qualifications in the various GCSE subjects, but Mr Gove believes this has led to a fall in standards; that they have competed to offer easier exams.
Under the changes coming in, exam boards will bid to be the only supplier of the EBC for individual subjects.
'Designing in the dark'
During the committee hearing, the chairman Graham Stuart said exam boards competing to win franchises to design replacements for GCSEs felt they were "designing in the dark", with not enough time.
The MPs heard that exam boards which want to bid to run individual English Baccalaureate Certificates in core subjects have to put their bids in in June, but would not hear the details of what was needed until Easter.
"They are designing in the dark, scared to complain about it in case you hold it against them," he told Mr Gove.
The education secretary told MPs he had not heard complaints from the exam boards about the brief they had been given and that they were aware of what would be needed.
He set out his reasons for introducing "more rigorous" exams, saying that although grades had been rising every year, standards had not.
He said there had been "grade inflation", fuelled by competition between exam boards to attract schools to their qualification, in effect that they had been making their exams easier.
The move away from modular exams towards those taken mainly at the end of two years of study would ensure there was better focus on teaching and learning and greater depth of study, Mr Gove said.
Asked why he chose to replace some GCSEs rather than reform them, Mr Gove said he had wanted "a clean break" and to move to a new system.
And he told MPs that he would be willing to overrule Ofqual after discussing their concerns.
"If they still had concerns and I still believe it is right to go ahead then I would do it, and on my head be it," he said.
Labour MP Pat Glass said the changes had brought "massive volatility" to the system.
Students starting GCSE courses in England this year will be the first to take their exams at the end of two years of study, rather than at various stages over two years.
Under plans announced earlier this year, students due to begin GCSE courses in 2015 will instead sit EBC exams in some subjects (English, maths and science)in 2017.
EBCs for other subjects - history, geography and languages - are due to be rolled out later.
Labour's shadow education secretary Stephen Twigg said it was time for Mr Gove to "think again".
"Michael Gove's exam plans are opposed by teachers, the CBI and by his own exams chief," he said.
"They understand they are narrow and out of date."
A spokeswoman for the Department for Education said: "We have been clear that the exams system is in desperate need of a thorough overhaul.
"That is why we want to introduce rigorous, robust and relevant exams for 16-year-olds, which will raise standards and give all young people the chance to succeed."
She added that the government was still holding a consultation on its plans and would respond to that in the New Year. The consultation closes on 10 December.