Teachers have expressed dismay that fewer than half the specifications for new GCSEs and A-levels to be taught in England from September are ready.
Updated figures from exams regulator Ofqual show 66 out of 156 of the new courses have been officially approved.
Labour's shadow education secretary Lucy Powell called the figures "alarming".
But education secretary Nicky Morgan insisted Ofqual was making progress, with draft content already available.
Under major exam reforms, new, toughened-up GCSEs and A-levels are gradually being introduced, with the second wave of courses due to be brought in for first teaching this September.
This includes key academic subjects such as the three sciences, history and geography, all part of the government's English Baccalaureate measure.
Suzanne O'Farrell, curriculum and assessment specialist with the Association of School and College Leaders, said subject content was available online but teachers also needed full specification details from each exam board, including marking schemes and sample exam papers.
"I do think this is a problem," said Ms O'Farrell.
"It is taking far longer than expected.
"As a head of department you want to make your choice of board from an informed position."
Schools also needed to make a curriculum plan, buy and read textbooks as well as setting up training for teachers in each subject, she added.
According to the Ofqual website some subjects are completely ready – for example art and design, computer science and music GCSEs.
However, no specifications are ready for chemistry, combined science, physics or biology GCSEs.
The position at AS- and A-level is equally patchy, with most modern languages and religious studies specifications still awaiting sign-off and only one board having been signed off for geography AS- and A-level.
The Association of Teachers and Lecturers called on the government "to get its act together and sort out the delay".
General secretary Mary Bousted said: "It's the second week of March and teachers are still waiting to find out what they are being expected to teach children in September.
"How are teachers supposed to prepare for teaching these new qualifications in September when they haven't had the opportunity to plan their curricula and develop materials based on the new specifications?
"And how will young people be able to decide which GCSEs or A-levels they want to study when they won't know until late this academic year what the content and assessment will be for the courses starting in September?
"This delay is particularly worrying because none of the science GCSEs have been accredited and several of the GCSE EBacc subjects have not been accredited."
'Flux and change'
Lucy Powell called the changes the biggest reforms in a generation and said they were "causing a huge amount of flux and change".
"We've been warning ministers for some time now that they are way behind in their timetable to deliver such a change."
Responding to a question from Ms Powell on the issue in the Commons today, Mrs Morgan said: "Ofqual are working with the exam boards to make sure that all of the specifications are ready…
"They are working their way through, as I say, to make sure that the exam boards and the specifications are ready to be published.
"We absolutely want to give teachers as much notice as possible."
Schools minister Nick Gibb later said the new exams represented "a new gold standard and it is right that the highest standards are applied to developing and accrediting these new qualifications….
"We have made clear to exam boards that they must produce high quality specifications as quickly as possible and will continue to work with Ofqual to ensure this happens."
An Ofqual spokesman said the website was being updated regularly.