GCSEs sat by hundreds of thousands of pupils this summer were too easy and suffered from inconsistent standards between the exam boards, the qualifications watchdog has told The TES.
Isabel Nisbet, Ofqual chief executive, admitted that the general GCSE science and additional science exams represented a "collective falling short of the standards that young people and teachers have a right to expect".
More than 800,000 pupils sat the qualifications this summer. Concerns about a lack of challenge posed by the exams in previous years had led the regulator to work with boards to tighten up and make them "more severe" for 2010.
But this week Ms Nisbet said: "If you asked me `Is GCSE science good enough?' I think the answer would be no."
Ofqual wanted to ensure the exams were tougher and that "disparities" in standards between boards were reduced.
Ms Nisbet said that on both counts "we are not quite there yet".
"I think we have made some progress in toughening up the standards and reducing the disparities but we still have a bit to go," she said.
This year saw large falls in the number of pupils taking the two combined science courses while there were rises of around 30 per cent each in the separate biology, chemistry and physics GCSEs.
The problems with science and additional science make it impossible to make reliable comparisons with results in past and future years, Ms Nisbet said.
The difficulties go back to 2008, when Ofqual intervened over inconsistencies between exam boards. Standards were supposed to be tightened in the existing exams this year, with new versions introduced for teaching from September 2011.
But Ofqual rejected boards' first new efforts in June and Ms Nisbet said the regulator would not accredit any qualification until she was sure standards had been met.
She said the problems were the collective fault of boards, regulators and ministers and the development of the qualifications being "rushed".
Correcting them had to be a "gradual process", she said. "You can't turn round a complete pantechnicon in one year because people already have modules in the bank.
"What (pupils and teachers) should know is that their exams have been fairly and consistently marked, rigorously checked, and therefore if they get a C or a B or an A, this a real achievement.
"They should not feel devalued by what has been a perfectly legitimate, finely tuned criticism at a level of sophistication which many countries would just not recognise."
Ms Nisbet would not give details of differences between the boards but said they were different to last year.
Both OCR and Edexcel said they are addressing Ofqual's criticisms in new specifications.
AQA, which says it was asked to lower its standards in 2008 to bring it in line with other boards, said it wanted Ofqual to take a "tougher stance".