The Association of Teachers and Lecturers have attacked the "haste" with which the Education secretary Michael Gove is proceeding with plans to overhaul GCSEs.
The teaching union said first year secondary school pupils were going to be "Mr Gove's guinea pigs".
We want all children to succeed in education, and we need exams that are rigorous. However, the haste with which Michael Gove is pushing through huge simultaneous changes to both exams and the curriculum carries major risks that will put last summer's English GCSE debacle into the shade.
"We particularly feel for the children in their first year of secondary school who are going to be Mr Gove's guinea pigs. They will have a single year being taught the new curriculum when they are 13 and then move straight into the new and untested GCSE exam syllabus at age 14."
Minister for education Elizabeth Truss defended the government's GCSE overhaul, insisting that it will lead to increased grades across the ability scale.
The minister said the new plans would help improve all students and raise grades among both high achieving and low achieving students.
The MP said that after comparing Britain to the rest of the world, the government had found the right approach for students through less coursework and more end of course exams.
English GCSE exams are to be made substantially tougher under controversial reforms expected to be announced today. Daybreak's Sue Jameson reports:
Russell Hobby, General Secretary of NAHT, has questioned whether or not the latest overhaul of GCSEs in England prepares students for the world of work.
He said: "Employers want students who can work in teams to solve problems, so I think this is quite a narrow vision of what education is for."
Pupils and parents will be concerned by the uncertainty thatMichael Gove has created around GCSEs. Having first talked down their value andthen failed in his attempt to scrap them, the Education Secretary is havinganother go at setting out his plans.
We need changes to assessments in schools that willstrengthen rigour and reflect the best ways of testing skills and knowledge.
Michael Gove has had plenty of chances to bring forwardevidence-informed policies but I fear he has not learnt from past mistakes. Hekeeps failing because he hasn't got a thought through plan to improve exams
Details of the most radical overhaul of GCSEs in England for a generation will be announced today, including plans to scrap the current grading system.
Exams regulator Ofqual will publish a consultation setting out proposals that are likely to include plans to axe coursework in the majority of subjects, an introduction of end of course exams, and less re-sits.
The grading system could also be overhauled leading to current A*-G grades being scrapped and replaced with a numbered system.
The Department for Education is also expected to unveil new information on the content of the exams today.
This report is clear that the problems in last year's English GCSE can be directly attributed to the design of English GCSEs, in particular the modular approach and the high level of controlled assessment.
The previous judicial review also came to that conclusion that it was the structure of the qualification that was to blame.
That's why we took immediate action to get rid of GCSE modules and are taking action to reduce controlled assessment.
The Education Committee is concerned that there is a rush towards separate exam systems for England, Wales and Northern Ireland, without careful reflection on what might be lost, or consensus that this is the right thing to do.
The turmoil surrounding last summer's GCSE English results highlights the importance of carefully developing new sets of exams.
A series of avoidable errors were made when the current GCSE English was being designed under the previous Government.
When pursuing future reforms, it is crucial that ministers and Ofqual pay careful attention to expert opinion and don't ignore warning voices.
The Education Secretary has announced plans to overhaul GCSEs and A-levels in England.
The reforms will see new GCSEs in academic subjects including English and maths introduced in 2015, as well as revamped A-levels in a number of subjects.
Michael Gove's proposals represent the most radical overhaul of examinations for 16-year-olds for a generation.
Last year, Mr Gove announced plans to replace GCSEs with English Baccalaureate Certificates, with each subject to be set by a single exam board.
However, this was later scrapped with Mr Gove conceding at the time "one of the proposals I put forward was a bridge too far".
MPs have raised concerns about any plans for separate exams systems in England, Wales and Northern Ireland, saying such a move would be "regrettable".
According to a new report by the Commons education select committee, all three nations should continue to run GCSEs and A-levels.
It also urged ministers to "do everything possible to bring this about".
The call, in a report into last summer's GCSE English controversy, comes just weeks after Education Secretary Michael Gove wrote to his counterparts in Wales and Northern Ireland suggesting that differences in exams reform mean that it is time for the countries to go their separate ways.
The cross-party group of MPs also said that ministers and England's exams regulator Ofqual must pay close attention to expert opinion on exams as they overhaul the system, and not ignore warning voices if concerns are raised.