England's exam system needs further changes, Education Secretary Michael Gove has told MPs, amid reports of plans to return to O-level style exams.
Mr Gove is reported to be preparing to replace GCSEs for England from autumn 2014 and also to bring in a simpler exam for less academic teenagers.
He was summoned to the Commons to answer urgent questions after details were leaked to the Daily Mail.
The Liberal Democrats are angry they were not told about the plans.
A senior figure told journalists: "We are very, very hostile to something that looks like it is going to return to the two-tier system of the past."
Mr Gove did not confirm the plans directly with MPs but praised many of the ideas, saying action was needed because the current exam system was letting children down.
"Children are working harder than ever but we are hearing that the system is not working for them," he said. "We want to tackle the culture of competitive dumbing down."
He said rigour needed to be restored to the system if England was to keep pace with educational improvements in some other countries.
Documents setting out the proposals for change were leaked to the Daily Mail and government sources told the BBC they were broadly correct.
The ideas, if introduced, would amount to the biggest change to the exams system for a generation. They are going to be put out for consultation.
The plan is for students to begin studying what the leaked document says will be "tougher" O-level style exams in English, maths and the sciences from September 2014. They would take their exams in 2016.
Less academic pupils would sit a different "more straightforward" exam, like the old CSE.
Labour's education spokesman Kevin Brennan told Mr Gove such a move would take the exam system "back to the 1950s".
"GCSEs may well need improving, but a two-tier exam system which divides children into winners and losers at 14 is not the answer," he said.
The Liberal Democrats said the plans appeared to set too low an aspiration for young people.
And a senior figure said changing the secondary exams system within two years could "lead to massive upheaval".
GCSEs replaced O-levels and CSEs in the mid-1980s. Under the previous system, the more academic teenagers took O-levels while others took CSEs (Certificates of Secondary Education).
News of the plans come as tens of thousands of teenagers across England, Wales and Northern Ireland finish taking their GCSE and A-level exams.
The leaked document also apparently shows plans for the national curriculum at secondary level to be scrapped altogether, so that heads would decide what pupils should study.
Already new academy schools, which are state-funded but semi-independent, do not have to follow the national curriculum.
And the government is said to be planning to scrap the traditional benchmark on which secondary schools in England are measured - the number of pupils getting five good GCSEs (grades A* to C), including maths and English.
Schools would continue to be measured on the government's new benchmark - the English Baccalaureate - which counts how many pupils in a school have good GCSEs in English, maths, two sciences, geography, history and a foreign language.
Another change suggested is that one exam board would be chosen to set the O-level style papers for English, maths and science - with all pupils taking the same exam.
Currently, six exam boards design GCSEs and schools choose which board to use.
It is this situation which Mr Gove believes has led to a "race to the bottom".
He told MPs: "We want to tackle the culture of competitive dumbing-down, by making sure that exam boards cannot compete with each other on the basis of how easy their exams are".
The suggestion that that has happened has always been roundly rejected by the exam boards and by the previous Labour government.
Critics of the existing system point to the year-on-year rises in the numbers of pupils achieving top grades as a sign that GCSEs have become easier, but supporters say teenagers are working harder than ever and teachers are getting better at preparing them for exams.
The government had already announced that it wanted to shake up GCSEs by returning to the system where most exams were taken after two years, rather than in modules, and those changes were already planned to affect pupils beginning their GCSE studies this autumn.
The big teaching unions have echoed Mr Brennan's warnings about a two-tier system.
Russell Hobby, the general secretary of the National Association of Head Teachers, welcomed the move towards having a single exam board per subject, which he said was sensible and would "remove a lot of concerns about the system".
"But a move to a two-tier system does not sound a good step forward," he added, saying such a change would mean choices about children's futures being taken at too young an age.
As control of education in the UK is devolved, Mr Gove's plans are for England only. It would be up to Wales and Northern Ireland to decide whether to follow suit. In Scotland, pupils take Standard Grades, Highers and Advanced Highers rather than GCSEs and A-levels.
The Education Minister for Wales, Leighton Andrews, has said Wales will not return to O-level-style exams.