The troubled saga surrounding the introduction of new science GCSEs continued this week as the exams regulator rejected the latest versions of the qualifications for being too easy.
Ofqual has told exam boards that their content is too basic, requires too little maths and does not stretch bright pupils enough.
The decision raises doubts over whether they will be ready for teaching in September 2011 as had been hoped.
Kathleen Tattersall, chair of Ofqual and chief regulator, said: "Ofqual's job is to make sure that standards are maintained. If qualifications do not meet our standards, we cannot accept them into the regulated system.
"Schools are expecting detailed information about the new qualifications in time to prepare for first teaching in September 2011.
Ofqual hopes that will still be possible, but progress will depend on the quality of the revised qualifications.
The problems go back to summer 2008 when Ofqual had to intervene to ask the Assessment and Qualifications Alliance (AQA) to lower the mark needed to achieve a C in the new GCSE science exam to ensure standards were consistent with those of other boards.
AQA did so "under protest", claiming its rival boards had made the exam easier - an accusation they denied.
The regulator asked all boards to tighten standards for GCSE science in 2009 to avoid a repeat of the controversy. But in November, Ofqual admitted that 2009 had seen further inconsistency between the boards on the exam.
In a stop-gap solution it asked boards to tighten standards further, but also ordered them to introduce replacement science GCSEs from 2011.
Now, having seen their efforts, Ofqual has told the boards to go back and try again.