The range of subject options available to secondary pupils is set to be dramatically cut as schools struggle to cope with imminent budget cuts.
Headteachers' leaders have advised schools to consider axing subjects taken by relatively small numbers of students as they are no longer cost effective, putting courses and teaching posts at risk.
The warning comes as the Association of School and College Leaders (ASCL) revealed that heads are rushing to sign up for new training sessions on how to make staff redundant as they prepare for tighter budgets.
The problems for schools wanting to offer a broad curriculum are likely to be considerably worse in rural areas where it is harder to pool resources and offer subjects such as foreign languages, music or design and technology at A-Level. Schools with small sixth forms could also be hit.
It follows the news last week that French - once considered a mainstay of secondary education - dropped out of the top ten most popular GCSEs taken this year. German also fell to a new low.
ASCL general secretary Brian Lightman said cutting expensive optional subjects will have to be considered by heads. It is likely that "the most able" students will suffer most, he warned.
"These kinds of management decisions that leaders have to make all the time will come more sharply into focus," Mr Lightman said. "Languages in some schools will be vulnerable. We are already worried about them and this could speed up the decline."
The narrowing of the curriculum has also prompted fears that less popular subjects could become the preserve of independent schools.
School budgets will come under increasing scrutiny as the Government announces the outcome of its comprehensive spending review in October. Ministers have said they will protect frontline services in education, but heads are preparing for cuts.
ASCL revealed that two out of its three courses on "managing staff reduction" are already full for this year and sessions on "strategic financial planning and staff deployment" have also generated significant interest.
Members have downloaded software designed to help reach decisions on making savings by juggling timetables and staff.
The association said the extra support is vital as the current generation of heads has not known a culture of cuts.
Andrew Jarman, head of Lancaster Royal Grammar School, a 1,000-pupil boys' state boarding school, said he was not able to offer music or Spanish last year because class sizes were not viable.
"Such `luxury' subjects, which aren't that cost-effective, are the ones we are now questioning," he said. "There has to be a drive towards real efficiency in staff and timetabling. We have noticed some set sizes in the sixth-form have grown to the low 20s, so how can you justify sets of five or six in other subjects?"
Mr Jarman said the school was offering A-level design and technology in partnership with a local girls' school from this term. "We are anticipating things will get tighter and tighter in the next three or four years," he said.
Universities have already expressed concern about the drop in those taking languages, and the Government is planning a review of the situation.
Linda Parker, director of the Association for Language Learning, said the impact could be particularly felt at A-level. "In straitened times, we urge heads to go the extra mile and offer subjects in collaboration."
Alan Clague, honorary secretary of the Joint Association of Classical Teachers, said it would be "a huge disappointment and a blow" if heads were forced to trim back Latin, which has experienced a surge in popularity in recent years.
"Classics generate a huge amount of passion in pupils that do it and numbers are self-generating once courses are established, but they need to be given time to gather momentum," he said.
John Bangs, outgoing head of education at teaching union the NUT, said cutting of optional subjects would lead entire schools to be "impoverished" and condemn children to an "arbitrary" curriculum.
"It will also leach children and talented teachers away to private schools, who have the money to spend on these subjects," he said.
- Original headline: Subjects face axe as cuts loom