Academies and free schools have been handed more than #163;26 million for thousands of sixth-form students who were never enrolled.
Figures uncovered by TES reveal that a group of academies and free schools, which are funded on the basis of predicted student numbers, received funding in 2011-12 for more than 4,700 students - the equivalent of 20 average-sized school sixth forms - that they never taught.
With the battle for post-16 students between cash-strapped rival schools, FE colleges and sixth-form colleges already more intense than ever, college leaders have warned that the projected funding allocations are giving academies an unfair advantage.
"It doesn't seem right," said David Igoe, chief executive of the Sixth Form Colleges' Forum. "That money should be spent on actual delivery, not on propping up uneconomic small sixth forms."
The Department for Education has admitted that not all the funding will be clawed back, with individual academies' contracts allowing them to keep between 2.5 per cent and 10 per cent of the excess funding. The Young People's Learning Agency, the funding body for 16-18 students, also admitted that it does not know how much has been reclaimed so far.
Academies that open new sixth forms are also given two years' grace to improve their student numbers before they are asked to repay any money, Mr Igoe said.
Julian Gravatt, assistant chief executive of the Association of Colleges (AoC), said that "bad contracts drawn up by somebody in government" were to blame for the confusion.
"Colleges have plenty of experience in sixth-form education and know that it's difficult to predict demand; some academy sponsors seem to think that it's easy and may have been overambitious," he said. "The lagged funding system (used for most of the newer converter academies) smooths out fluctuations a bit by calculating 16-18 funding for both colleges and schools on the basis of last year's recruitment. This is a good system, but some academies have special deals that mean they get 16-18 allocations on the basis of longer-term plans agreed a few years back, rather than on the basis of last year's recruitment."
David Wood, principal of Lancaster and Morecambe College, which is feeling the pressure from neighbouring academies' sixth forms, said that this means the costs per student are "extravagant".
"Why should these academies have a distinct advantage? Some of this extra cash will have been taken from the budgets of colleges, which are expecting student numbers to drop as a result," he said. "It's creating an unfair competition. They will be able to offer their students a potentially better experience because they are funded better."
A DfE spokeswoman said: "A small proportion of academies - mostly older sponsored academies - receive funding based on pupil estimates, not actual pupil numbers."
A report published by the AoC last week revealed that the battle for students between schools and colleges is growing increasingly bitter. The survey found that 51 per cent of schools with their own sixth forms offered "poor, limited or no access" to information about their local FE college, compared with just 14 per cent of 11-16 schools.
The research found that the most common problems included schools restricting pupils' access to college liaison officers, refusing to distribute prospectuses and barring pupils from attending college "taster" events.
The discovery that #163;26 million is sitting in academies' coffers for students that they never enrolled will do little to ease the tension.
75,444 - The predicted number of students that the group of academies and free schools received funding for in 2011-12.
70,702 - The actual number of students taught by the schools in 2011-12.