The attack from the Educational Institute of Scotland (EIS) teaching union follows the publication of new agreements between colleges and the Scottish Funding Council (SFC).
An EIS spokesman said: "The outcome agreements show funding for college teaching is being cut, with a reduction in funded teaching activity, leading to fewer students."
However, John Henderson, chief executive of Scotland's Colleges, said the agreements showed the crucial role colleges were playing.
"At a time of high youth unemployment, colleges are equipping young people with the skills and qualifications for work and, as these outcome agreements show, some 70% of college activity is now focusing on 16 to 24-year-olds," he said.
Meanwhile, the Scottish Government said a college programme run by Skills Development Scotland had found places for almost 1500 young people in courses for improving job prospects.