England's qualifications watchdog, Ofqual, has warned the Westminster government that its reforms to GCSE exams for 16-year-olds must be "thought through carefully".
Michael Gove, the education secretary, announced plans this week to replace GCSEs with new qualifications called English Baccalaureate Certificates (EBCs) from 2017.
Ofqual's chief executive, Glenys Stacey, responded immediately by writing to the Department for Education saying Ofqual would advise it on whether the timetable for change was achievable.
Under the changes, now being put out for consultation, pupils starting English, maths and science courses in 2015 will be studying for the EBCs and sitting them in 2017; in 2016, an additional three core subjects will be added - history, geography and languages - to be sat from 2018.
Many GCSEs will continue to be sat after 2017, as the EBCs will only cover six core subject areas initially.
Ms Stacey's letter said standards must remain the top priority, and exam boards and schools must have a "good enough understanding of what is expected".
She added: "We will advise government on the timetable for change, and say if it is not achievable or if the risks to standards or delivery are unacceptable.
"We know from recent experience that reforms to qualifications need to be thought through carefully (we will report on our findings in relation to GCSE English qualifications next month). We will wish to identify the delivery pressure points in the reform of GCSEs, and intervene if we need to in order to manage any unacceptable risks."
The EBCs will end "grade inflation and dumbing down", said Mr Gove.
The consultation paper, published earlier this week, set out the basic principles for the reforms: the new qualifications are designed to be far more dependent on performance at the end of a two-year course than on pupils' results in modules or units along the way; and they will allow for fewer resits. A return to the O-level system by which grades were decided on how a fixed percentage of candidates performed, rather than what individual candidates knew, will not now happen.
Mr Gove's reforms won Liberal Democrat backing after agreement that the EBC would be universal and not part of a two-tier system.
But Stephen Twigg, Labour's education spokesman, warned that a future Conservative government might still introduce a second exam alongside the EBC for less able children. His party would reverse any move towards a two-tier system and would reinstate coursework, he said.
Ringing the changes: from GCSE to EBC
GCSE More than 60 subjects
EBC Six core academic subject groups: maths pure and applied - with an additional maths option; English language and literature; sciences - chemistry, biology and physics (2015); languages - with flexibility on oral exams; history; geography - field trips will continue to count (2016).
GCSE - Three large boards (AQA, CIE and Edexcel) and other smaller ones.
EBC - Single board per subject for five years, decided by tender.
GCSE - Modules with exams for each and assessed coursework.
EBC - No coursework, single set of longer exams at end of course.
GCSE - Pupils can resit each module to bump up marks.
EBC - Pupils would have to resit the whole exam.
GCSE - Calculators, periodic tables, source materials allowed.
EBC - Aids restricted wherever possible.
GCSE - Eight grades from A* to G.
EBC - New grading structure, perhaps 1-10, and more detail.
GCSE - Choice of higher or foundation tier, with maximum grade C.
EBC - Students could study for longer and sit exam at 17 or 18.