The school exams revolution announced this week will lead to more "wrong grades" and fluke results, according to both the academic whose research is being used to justify the changes and a government education adviser.
Education secretary Michael Gove is planning new English Baccalaureate Certificates (EBCs) in English, maths and sciences, to be sat from 2017 by all pupils who would have taken the GCSEs they will replace.
The tougher qualifications will largely dispense with coursework and modules, with the aim that two years of "deep" learning will be assessed in each subject by a single "all or nothing" three-hour exam.
But Professor Robert Coe, whose work on GCSE grade inflation was cited by the government as it unveiled the EBC plans, said it would be "near impossible" to reliably test such a wide range of ability in a single exam.
"I don't know what they are thinking or who they have talked to," the Durham University academic told TES. "Where are the models for this?"
Both he and Professor Dylan Wiliam, a member of the government's National Curriculum Review expert panel, have independently concluded that the new qualifications will lead to results more dependent on "luck".
Professor Coe's research, which suggested that GCSE standards had fallen, is being used by the government to back its calls for reform. But the academic said the broad range of ability of pupils taking one exam would limit the amount of the syllabus that could be tested. "It also means that the precision, the reliability of the scores is less," he said.
"The luck of the draw - whether you get questions that you happen to be able to do or you can guess or whatever - starts to play more of a role. You get more wrong grades. That would be the outcome."
The prospect of further grading problems is likely to raise particular concerns following this summer's GCSE English controversy.
It is understood that Mr Gove originally wanted his EBCs to emulate Singapore, where about 80 per cent of pupils eventually take academic ^O levels, and the rest sit less demanding exams set at a similar standard to England's old CSEs.
But Liberal Democrat opposition to a "two-tier" system when the plans were first leaked in June has led the lower ability exams to be dropped. The agreed coalition position means that the new exams will now not only have to toughen up GCSE standards, but also accurately measure the performance of pupils of all abilities.
Professor Wiliam doubts whether they can reliably work. "The scores that students get will just depend on the luck of the draw," he said. "This is unavoidable because it is beyond the ability of test writers to write enough good questions every year that could actually differentiate by outcome."
The proposal for single three-hour exams, contained in a leak last weekend, is not specifically repeated in the official consultation. But a source close to Mr Gove confirmed that although there might be exceptions for oral language exams or practical science work, the "all or nothing" exam was the main aim.
The two academics said this would be particularly hard to achieve in subjects such as science or maths. Professor Coe said the use of different tiers of exams was the only way he could see it working. Mr Gove has specifically ruled this option out.
The pass mark for EBCs will be more demanding than a GCSE C grade. The new exams, likely to be graded numerically, will also have lower grades below a pass.
Mr Gove accused those who feared that more would fail the new exams of "fatalism".
But Professor Coe said that taking the tougher exams would be "soul destroying" for lower ability pupils. He also warned that they could be doubly disadvantaged by not being "clever enough" to spot the exam questions appropriate to their ability.
The proportions of candidates achieving each EBC grade will not generally be fixed, as some had feared. But a source close to Mr Gove suggested that any exam board proposal for limiting the very top grade in a qualification to a fixed percentage of pupils might be considered.
First teaching will not begin until 2015, allowing Labour to axe the plan if it wins the next general election. Stephen Twigg, shadow education secretary, suggested it could be a Conservative Trojan horse for an eventual two-tier system.
A Department for Education spokeswoman said: "It will be for the awarding organisations to decide what form the exams will take. We have just opened a consultation and will be discussing precisely these details with experts."
- Tougher English Baccalaureate Certificates (EBCs) in English, maths and the sciences will be introduced from 2015.
- EBCs in history, geography and ancient and modern languages will follow.
- Pupils who fail at 16 will be expected to try again in later years and will receive statements of achievement in the interim.
- New floor targets for schools will be introduced from 2017 when the exams are first sat, and a consultation on changing school league tables will begin later this year.
- GCSEs will coexist for the first years of the EBC at least, but Ofqual will be asked to use the EBCs as a template for GCSE replacements in other subjects.