Sixth-form colleges in England say they have had to cut the number of science and foreign language courses they offer, because of financial pressures.
The Sixth Form Colleges Association also said more than a third of colleges feared that without better funding they could cease as going concerns by 2020.
It sent online questionnaires to all 93 colleges; 72 responded and of these 26 said they feared for their viability.
Last month the government announced a review of post-16 education.
Sixth-form colleges have faced deeper cuts to their budgets than any other group of institutions, the association's report said, with some losing a third of their funding between 2011 and 2016.
The future is "equally bleak", as the government has decided not to protect the 16-19 education budget from spending cuts.
"Further reductions are highly likely," the report said.
In addition, sixth-form colleges are not eligible to reclaim VAT, meaning institutions have lost out, on average, to the tune of £317,964, the report said.
The Department for Education said moving to provide funding on a per-student basis had ended unfair differences between schools and colleges.
The association's questionnaire asked each college to report the cumulative impact of the funding cuts introduced since 2011.
Of the 72 colleges that responded, 52 said they had dropped courses as the result.
A-levels in modern languages have been cut in 28 colleges - more than a third - while 17 - just under a quarter - reported cuts in science, technology, engineering and maths subjects.
Colleges "have responded creatively and decisively" to the financial squeeze, says the report - but 68 out of 72 said they were "concerned" or "extremely concerned" about their financial health.
Asked about the year 2016-17, 50 colleges believed their funding would not be enough to provide a high quality education, while 59 said they would not be able to provide support for disadvantaged students.
Sixth-form colleges outperform school and academy sixth forms while educating more disadvantaged students and receiving less funding, says the report, but it continues: "The sector has reached the point where it cannot absorb any further reductions."
Sixth Form Colleges Association deputy chief executive James Kewin said funding inequalities between sixth-form colleges and school and academy sixth forms should end.
"The sector cannot survive on starvation rations... and will be unable to provide young people with the high-quality education they need to progress to higher education and employment," he said.
"A major reform of post-16 education and training institutions is now necessary," said the announcement.
"We will need to move towards fewer, often larger, more resilient and efficient providers."
The review will focus on further education and sixth-form colleges, "although the availability and quality of all post-16 academic and work-based provision in each area will also be taken into account".
The announcement also promised early action on financial stability, with possible solutions including asking colleges to review market position, financial management and informal twinning arrangements with other institutions.
A Department for Education spokesman said the government had "ended the unfair difference between post-16 schools and colleges by funding them per student, rather than discriminating between qualifications.
"We have provided sufficient funds for every full-time student to do a full timetable of courses and increased support for those who successfully study four or more A-levels and large TechBacc programmes."
Thousands more students now stay in education or training after the age of 16, the spokesman added.
Tristram Hunt, Labour's Shadow Education Secretary, called the findings "deeply troubling".
"Modern languages and science are an essential part of a 21st-century curriculum. We need to widen young people's horizons, to equip them with the broad range of knowledge and skills they need to thrive in the new digital world.
"Once again this shows how David Cameron's education policy is holding Britain back."