Plans to axe GCSEs in England could fail to help less able pupils and leave some subjects with discredited qualifications, MPs have warned.
The Commons Education Select Committee agrees that significant improvements are needed.
But it claims the Government has failed to prove its case for scrapping the qualification.
GCSEs will be replaced by English Baccalaureate Certificates, with teaching beginning in 2015, and the first exams in English, maths and sciences taking place in 2017.
GCSEs in history, geography and languages will be replaced by EBCs at a later date, and GCSEs are set to remain in other subjects.
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Committee chair Graham Stuart warned ministers they could be being too hasty.
"Slow down a little. Don't allow educational reform, which has impact for generations to come, don't let that be driven by a political timetable."
Education secretary Michael Gove has said the changes will modernise the exam system.
But Jacques Szemalikowski, head teacher of Hampstead School in north London, is not so sure.
"The Secretary of State says the idea of a risk is not a reason not to proceed, well I agree with that, but at the same time we need something that is coherent, that has a logic and an underpinning philisophy, in terms of making our education system compete with the best, and not back to the future."
English teacher David Robson is worried the Government could be gambling with children's futures.
"If you've got some students and everyone a year older than them have done a different qualification, where does it put those students when going into the job market? Where does it put them now in terms of preparing for the future?
"So I think it's unsettling in many ways, and obviously any year group who are going to be the first wave in any system, in a sense you're going to be the guinea pigs."
The committee said there was a "lack of coherence" about the Government's approach to reforming the curriculum, qualifications and the school accountability system.
It warned that the reforms could have a negative impact on subjects that will remain GCSEs.
It also suggested the Government should focus on improving the achievement of the "significant minority" that do not achieve five good GCSEs, rather than considering plans to introduce a Statement of Achievement for lower-attaining pupils.
A Department for Education spokeswoman said: "We have been clear that the secondary education system is in desperate need of a thorough overhaul - an objective with which the committee agrees.
"That is why we are making major changes to ensure we have world class exams that raise standards."