Examinations boards have been ordered to redraft new science GCSE papers because they are not rigorous enough.
The exams watchdog, Ofqual, said the new papers - designed to address concerns that science exams had become too easy - had "not gone far enough".
Last year Ofqual said science GCSEs taken in 2007 and 2008 had contained too many multiple choice papers and had failed to challenge the brightest.
Improvements have already been made to this year's paper, Ofqual said.
Ofqual previously ordered an overhaul of GCSE science qualifications and immediate action was taken to toughen them up for students sitting them last year and this year.
Now the watchdog says the new-look qualifications, due to be introduced in autumn 2011, have been sent back to the exam boards for more work.
Chair of Ofqual Kathleen Tattersall said: "I look forward to receiving improved GCSE science qualifications that meet our requirements.
"Schools are expecting detailed information about the new qualifications in time to prepare for first teaching in September 2011.
"In the meantime, improvements have been built in to the current version, and the regulators will make sure that grades awarded this summer are appropriate and fair."
Ofqual is baring its teeth after the qualifications and curriculum agency, the QCDA, was scrapped by Education Secretary Michael Gove.
A spokesman for the exam board OCR said: "OCR is naturally disappointed that Ofqual did not accredit its GCSE science specifications.
"They were designed by experienced syllabus developers in close contact with the science community.
"However, given that they were built to QCDA criteria, this ruling clearly indicates that Ofqual had major problems with its partner quango - and that the government was right to scrap it.
"OCR trusts that Ofqual will now reach a new level of transparency about what is required from awarding bodies and will start work on amending the syllabus immediately."
Single exam board
A spokesman for the exam board AQA said: "We are addressing the issues that Ofqual has raised, and will be re-submitting our specifications for accreditation, whilst maintaining the innovations that teachers and subject communities have told us that that they value.
"Teachers and students can be assured that these new specifications will be ready in time for first teaching in 2011."
Chris Keates, general secretary of the NASUWT, said: "This latest development only highlights the inefficiency of having several awarding bodies struggling to interpret the requirements of the regulator, Ofqual.
"The government continues to emphasise the importance of financial efficiency due to the economic climate.
"If this is the case, ministers should be giving serious consideration to creating a single awarding body, firmly located within the public sector and fully accountable for its actions.
"This will ensure greater consistency in the qualifications system as well as providing better value for money."
Christine Blower, general secretary of the National Union of Teachers, said: "Ofqual's actions to investigate the new science GCSEs confirm that pupils, parents and employers can be confident that all other GCSEs continue to uphold high standards."