LONDON GCSE science exams taken by more than 500,000 children aged 15 and 16 last summer failed to stretch able pupils and contained too many multiple choice questions, the government's exams watchdog said on Friday.
Ofqual said a review of the specifications for the science, additional science and physics exams raised "significant causes for concern."
The findings were seized on by critics who argue GCSEs have been "dumbed down" and no longer challenge the ablest pupils.
The regulator ordered exam boards immediately to improve the quality of questions and the rigour of marking in time for this summer's tests.
And it said the entire science specification should be rewritten in time for exams in summer 2013.
"Our monitoring shows that the revisions to the GCSE science criteria in 2005 have led to a fall in the quality of science assessments," said Ofqual Chairman Kathleen Tattersall.
"Science is a vitally important subject and it is essential that these new criteria and specifications should engage and challenge all learners, particularly the most able."
Many private schools have switched to teaching International GCSEs, designed for overseas pupils and which offer a more traditional syllabus and test.
"Even those accustomed to reading between the lines and de-coding quango-speak will be shocked by these findings," said Geoff Lucas, secretary of the Headmasters' and Headmistresses' Conference, which represents 250 independent schools.
"Any one asking why so many independent schools are switching to the IGCSE, particularly in science, only needs to read this latest Ofqual report."
Earlier this month Children's Secretary Ed Balls criticised independent schools for claiming IGCSEs were the only way to stretch the brightest pupils.
"The idea that you have to look at private schools teaching the IGCSE if you want to see pupils being really stretched isn't just out of touch with the reality of our education system, but it also undermines the brilliant work being done by many of our best school leaders," Balls told a teaching conference.
The opposition Conservatives said report confirmed the government had devalued science exams.
"A Conservative government would reverse the devaluation of exams and make them world-class again," said Conservative Schools Spokesman Nick Gibb.
Schools Minster Jim Knight said he was concerned by the Ofqual findings but stressed that the GCSE exam system was sound overall.
"This is a science problem not a GCSE problem -- I am reassured by Ofqual's findings that "the system is generally in good health'," he said.
The criticism comes a month after Prime Minister Gordon Brown pledged to double to 100,000 the number of pupils in state schools taking three separate science GCSE exams in Physics, Chemistry and Biology, by 2014.
The Ofqual complaints focussed on two new science exams, GCSE science and additional science, taken for the first time last summer and aimed at pupils not taking the more demanding "triple science" option.
Ofqual said it was also concerned that the physics GCSE course had become less demanding.