Sixth form colleges have dropped courses in sciences and languages amid fears the system is "under serious threat" from funding cuts, according to a new survey.
The online poll, carried out by the Sixth Form College Association (SFCA), also revealed a third of college leaders believe their institutions will not be able to operate after 2020 without more investment.
The findings, published in the SCFA's Funding Impact Survey report, prompted the group to call for an urgent review into education funding.
The report warned: "The Government's decision not to protect the 16-19 education budget from spending cuts means that further funding reductions are highly likely."
Sixth forms, the body said, had suffered deeper budget cuts than other institutions since 2011 as a result of changes such as reduced entitlement funding for extra-curricular activities and a new 16-19 funding formula which had seen the average college lose 6% in funds. Some had lost as much as a third of their budgets, the report added.
SCFA deputy chief executive James Kewin said: "The sector cannot survive on starvation rations and without more investment sixth form colleges will be unable to provide young people with the high-quality education they need to progress to higher education and employment.
"The Government should conduct an urgent review of funding across all stages of education and end the funding inequalities that exist between sixth form colleges and school/academy sixth forms - particularly the absence of a VAT refund scheme that, according to our report, left the average sixth form college with £317,964 less to spend on the front line education of students last year."
The survey was sent to all 93 sixth form colleges in England and of the 72 that responded, 72% said they had been forced to drop courses since 2011.
Some 39% had stopped offering modern foreign language A-levels, such as French, German and Spanish, while 24% cut science, maths and engineering options.
More than three quarters of colleges said they had reduced or removed extra-curricular activities such as sport and drama, while four out of five said class sizes had increased as a result of cuts.
Disadvantaged pupils would be "disproportionately affected", the report concluded, as 83% of colleges said they did not believe funding levels for 2016 would allow them to offer the support required by those students.