Further education colleges fear the Scottish Government's guarantee of training places to people aged 16 to 19 will leave no money to fund around 105,000 places for older students.
They have also predicted a net loss of 20,000 full-time equivalent college places across Scotland as a result of a 20 per cent budget cut over the next three years.
But those claims were rejected by Education Secretary Michael Russell, in a performance described by political opponents as "utterly dismissive".
Scotland's Colleges, in evidence to the Scottish Parliament's education committee, said that even if they took a quarter of the 16 to 19 age group not in education, employment or training, and continued current arrangements for 18 to 24-year-olds, "there would be no funded provision left for adult learners, who make up over half the number of students we serve".
There are 105,000 Scottish Funding Council (SFC) funded adult learners at present.
The colleges told MSPs that more Government funding would be needed to keep "the range of opportunities for those over 24, who are often in need of support and essential training to re-enter or improve their prospects in the workplace".
At the education committee, Mr Russell described the prediction of 20,000 lost places as "speculation", since colleges could only establish figures through the tripartite arrangement with the Government and the SFC, and at present there was no agreement.
Mr Russell also dismissed suggestions by Angus College principal John Burt that his institution could lose 2,000 places, because no such decision had been made by the Scottish Funding Council.
He reiterated that training places would be provided for every person aged 16 to 19 who wanted one, which could entail apprenticeships and higher education as well as colleges.
He criticised the "astonishingly poor performance" that resulted in 28 per cent of college students dropping out or failing to achieve the qualifications they had initially aimed for, and raised the prospect of incentives for colleges to improve staying-on rates.
The 20 per cent cut in college budgets would not help open up education to a broader range of people, Mr Russell admitted. He accepted that the Government was partly responsible for not making enough progress on widening access to further and higher education; the Government would seek to legislate on this issue.
He was less contrite about compulsory redundancies: although he warned college principals earlier this year that they should not resort to such measures, he stressed this week that he could not tell colleges what to do.
Labour education spokesman Ken Macintosh criticised Mr Russell for repeatedly citing Scottish independence as the salvation of college funding.
"Mike Russell's dismissive attitude to criticism and close questioning is not the way to protect Scotland's further education sector," he said.
Liberal Democrat education spokesman Liam McArthur described Mr Russell as "utterly dismissive" about the impact on colleges of pound;70 million of cuts.
Mr Russell appears to be putting pressure on Edinburgh's Telford College to merge with fellow city FE colleges Stevenson and Jewel and Esk.
Telford pulled out of the merger plans earlier this year, but Mr Russell told the education committee Telford would be "enhanced" by joining forces with the other two.
Labour FE spokeswoman Claire Baker said that if she were at Telford College, she would be "concerned", and accused Mr Russell of wanting to "manipulate the FE map of Scotland".
Original headline: Adult learners could lose out to 16-19 focus, colleges warn