Class sizes in colleges could rise to an average of 27 students or higher if FE cuts go through, the largest union warned the Coalition in its spending review submission.
The University and College Union (UCU) said cuts of 25 per cent over the four years would see college student-teacher ratio rise from one in 19.9 to one in 27.6. In adult education, the expected cuts could be even more damaging.
The UCU described its figures as a "conservative estimate" and warned job losses could be even worse because other college overheads were usually fixed.
The union was urging the Government to maintain FE spending at 0.6 per cent of the economy, equivalent to last year's spending.
"The Government needs to fund adult learning," it said. "The UK already lags behind many other western countries in educational expenditure and it is interesting to note the emphasis placed on investment in education in newly dominant economies like China.
"We cannot afford the economic and human cost of weakening our education system at precisely the moment we need to strengthen it to meet the demands of the future."
Expectations that business or individuals could make a bigger contribution to adult education were misguided, UCU argued, as the recession has left both less able to pay.
Employer expenditure on training had fallen 5 per cent since the start of the recession in 2007, according to UK Commission for Employment and Skills figures.
"The likely impact of the recession means Government funding is paramount, especially in the development of new jobs in high-skill, sustainable- development and low-carbon industries," UCU said.
Shona Terry, a course leader at York College, said: "Further education is one of the major stepping stones to reaching higher education for learners who are not the highest fliers. The funding of this kind of education needs to be kept within the public sector. "
The union also argued to keep the Education Maintenance Allowance, describing it as a powerful incentive for post-16 students to stay in education. Before the election, Michael Gove said the Conservatives would not scrap the pound;30-a-week allowance.