Students are discouraged from broadening their studies because of the stiff competition for cash between college departments, research shows.
Timetable problems restrict student choice despite efforts to target cash at programmes which offer a mix of academic and vocational studies, according to the research from the London University Institute of Education.
The findings have serious implications for the Curriculum 2000 programme that aims to encourage all post-16 students to study five rather than three subjects from this autumn.
Ministers expect A-level students to consider a wider mix of arts, humanities and science. All students, vocational and academic, will be urged to undergo key skills training in their weak areas, such as literacy, numeracy or information technology.
But Tony Haines, a researcher, reveals that thee are still considerable difficulties producing a cross-college timetable and that these could grow as colleges attempt to increase choice.
"The college timetable acts as a filter, reducing access to often unrelated studies," he said.
Additional funding also acts as an incentive for departments to encourage students to stick with the extra courses they offer.
The pressures within a college do lead to unintentional benefits, since students are more likely to gain a coherent programme of studies, but it reduces the chance of reaching a balance across a wider range of subjects throughout the college.
Instead of seeking to negotiate a range of studies for each individual, "these departments seek to counsel students into undertaking additional studies in the same department as their main programme of study", he said.