Schools and colleges are to snub the Government's plans for a five-subject sixth form.
A powerful coalition of independent and state-school headteachers this week condemned the long-awaited scheme for increased sixth-form breadth as ill-conceived and unworkable - even if free periods and courses in key skills and general studies are abolished.
The group, which represents every secondary school and college in England and Wales, says there aren't enough hours in the week to cope and claims to be "amazed" that ministers have ignored the past two years' professional advice.
Last month the Government said it wanted a wider range of sixth-form studies, with students encouraged to take three A-levels plus two, smaller, AS-levels.
But the the Joint Associations Curriculum Group, describes this as "an impossibility" for school and college timetables and says they will end up sticking to the current, restrictive, norm of three A-levels per student.
The Department for Education and Employment this week hinted that schools should respond by lengthening the sixth-form day, putting it in line with continental norms of around 30 taught hours a week.
The JACG has played a key advisory role in the past two years' debate on post-16 qualifications. It is made up of the Secondary Heads Association, the Headmasters' and Headmistresses' Conference, the Girls' Schools Association, the National Association of Head Teachers, the Association of Heads of Independent Schools and the Association of Colleges.
"I don't understand how these proposals came about," said Dr John Dunford, group spokesman and general secretary elect of the Secondary Heads Association.
"We told the Government that they were unworkable, but they seem to have forgotten our advice. I was amazed to see them on paper.
"It's quite unfeasible for anybody to study five subjects in the way they suggest. There are not enough hours in the week."
Alternative proposals for two A-levels and three AS-levels (rather than three As and two AS-levels) would, in contrast, be workable, he said.
"If students in the lower sixth study five subjects for five hours each, that would mean they'd be in contact with the teacher for every minute of the week. It would be quite impossible.
"There would be no private study time - a very important part of university preparation. It would also abolish any other courses, key skills or citizenship, for example."
Instead, he said, students will end up as now studying the same three subjects all the way through their sixth form or college.
A government source pointed out that 30 hours of taught time is the norm in continental Europe, substantially more than in Britain. "I hope schools would take that into account," he said.