A report on the study will suggest college managers are withholding figures on the scale of the problem fearing cash penalties under the complex spending formula of the Further Education Funding Council.
The research by the National Institute of Adult Continuing Education indicates that the number of adults quitting courses is almost double that shown by official figures.
The report called Staying or Leaving the Course underlines claims that the Government's cost-cutting to improve efficiency is becoming counter-productive.
Services being whittled away include transport and student support. The NIACE research backs findings by the Association for Colleges that cuts are hitting adult education the hardest.
But the AFC is sceptical about the 50 per cent drop-out rate claim. Dr Veronica McGivney, author of the report and NIACE research and development officer, warns: "It is extremely difficult to obtain a reliable overall picture. The national database is inadequate and institutions define and measure non-completion in a variety of ways."
The report's findings will be embarrassing for ministers who this week launched the UK programme for the European Year of Lifelong Learning in Edinburgh. Dr McGivney also says that colleges are giving inadequate information.
"Because of the financial implications, many institutions are reluctant to reveal their withdrawal rates which vary widely according to the institution, student cohort, subject area, type of course and learning mode," she adds.
The NIACE study shows that adults are no more likely than school leavers to quit because of outside commitments, instead they were often more motivated.