Four out of every 10 full-time college students drop out of their course or do not get as far as they had hoped with their studies.
Scottish Funding Council figures show that 28 per cent of full-time further education students quit their courses in 2009-10, while 12 per cent completed their studies but did not get the full qualification they were aiming to achieve.
"These drop-out rates are very disappointing," said Graeme Kirkpatrick, depute president of the National Union of Students Scotland.
"While some students leave their course for good reasons, like getting a job, we know that often dropping out can be down to financial reasons or difficulties with the course."
He predicted that drop-out rates would rise again if the Scottish Government made any more funding reductions; this would result in "a huge waste of talent".
The Scottish Funding Council suggested that the figures reflected colleges' efforts to attract people who otherwise might not have considered further education.
"To an extent, the statistics reflect the challenges faced by colleges reaching out into communities and trying hard to get people involved in learning," said chief executive Mark Batho, who stressed that the funding council would be "working closely" with colleges to improve retention rates.
Data was collated in a new way that precludes comparisons with previous reports; this year, the SFC counted drop-outs from the very beginning of courses rather than waiting until a quarter of the way through.
The funding council is also concerned that courses which do not lead to recognised qualifications may have become too prevalent, with the figures showing they equated to 7 million hours of learning.
The Scottish Government commissioned a review of post-16 education and vocational training which last month called for the way the sector is financed to be overhauled. The Government said colleges continued to perform strongly and were supported by record levels of funding.
Henry Hepburn, email@example.com.