Headteachers’ leaders are dismayed at the decision, saying the proposals to broaden sixth-form studies could cost up to pound;600million. They warn that the quality of state sixth-form education will be jeopardised unless extra cash is found.

They fear that schools will be forced to cut teaching time per subject, reduce the number of subjects and increase sixth-form class sizes to unacceptable levels.

However, education minister Baroness Blackstone told The TES that many sixth-form classes are too small and would be boosted by extra students.

Schools had benefited from substantial extra resources and were in a strong position to deliver more subjects to more students without more money, she said. But headteachers argue that post-16 funding has suffered severe cuts, making it impossible for schools to implement the reforms without threatening quality.

Under the changes to post-16 education, sixth-formers will be expected to take up to five subjects in their first year.

The Government hopes the proposals, which include new AS-levels, modular A-levels and revamped GNVQs, will encourage more pupils to stay on and a broader range of subjects.

But headteachers are concerned the extra teaching load could cause massive staffing and timetabling problems. Arthur de Caux, senior assistant secretary of the NAHT, said: “Without extra funding the whole thing is quite impossible. It is beyond belief that ministers think there is any slack in the system to fund this amount of extra teaching in schools.”

John Dunford, general secretary of the Secondary Heads Association, said:

“While we welcome the reforms in curriculum terms, we are very concerned about the costs and funding levels in post-16 education.”

Baroness Blackstone, speaking after a conference on post-16 education, said there would be no funding to support the extra teaching load. “I am well aware that groups in maintained sixth forms are very small,” she added. “A few more pupils would strengthen these groups.”

She denied the reforms would be harder for smaller sixth forms to implement. “As far as I am aware there are no schools only offering three A-level subjects, so I see no reason why any school should have a problem. They may find it helpful to work with other schools.”

Baroness Blackstone had addressed more than 250 independent and state school headteachers at a joint Headmasters’ Conference and Girls’ School Association conference in Leeds on the future of sixth-form studies.

Delegates, including heads of leading public schools, were united in their rejection of plans for five subjects in the lower sixth. This, they said, would squeeze out all extra-curricular activities and should be attempted only by a few bright students.

James Sabben-Clare, HMC chairman, said he was disappointed no traditional linear A-level syllabuses would be retained under the reforms.