Exam boards are being ordered to rip up their new science GCSEs because of concerns from the qualifications regulator about reliability and the number of resits allowed.
Ofqual also revealed today that physics exams have become easier.
The regulator's two reports will trigger alarm about the subject. It looked at science and additional science GCSEs, introduced in 2006 and allowing multiple resits. In one of these, candidates can sit up to 48 different tests with only the best nine results counting.
Ofqual concluded that there were "a number of concerns regarding the validity and reliability of different assessments" used.
The regulator has commissioned new criteria for science that will only allow one resit per unit. This will lead to the boards developing replacement qualifications that can be introduced from 2011.
Kathleen Tattersall, chair of Ofqual, said: "The science reports are clearly a cause for concern. It is absolutely essential that standards remain consistent from year to year and across awarding bodies."
The regulator also reviewed standards in physics and concluded that between 2002-2007 there had been an overall decline in performance at every grade of the GCSEs offered by the three main English boards. "In some cases the decline was very marked," Ofqual said.
For A-level physics, the regulator reported a decline in standards at A- grade between 2001 and 2007. It was particularly marked for OCR candidates. At grade E, there was a decline in performance for OCR and AQA candidates, but Edexcel E-grade candidates were judged to be stronger.
In GCSE physics it found trivial questions not linked to knowledge or understanding of physics principles and a reduction in content.
Important concepts were fragmented across different modules and there were shorter, easier questions, fewer extended written responses required and a lower mathematical requirement.
On the science GCSEs, Ofqual found the type of assessment, and weight given to it, varied significantly between boards, making comparability in grades difficult.
The new science GCSEs were already controversial after it emerged that Ofqual had asked AQA to drop its grades to bring then into line with other boards last summer, prompting accusations of dumbing down.