Nearly 34,000 full-time jobs are at risk in colleges, more than half of which could be front-line teaching posts, according to figures from the University and College Union (UCU).
Of the 33,800 full-time equivalent (FTE) positions under threat, almost 18,800 are teaching jobs, the union said today.
But the actual number of people who may lose jobs could be significantly higher since FTE figures obscure the part-time headcount.
Redundancies on this scale would see the student-teacher ratio rise from just less than 20:1 to higher than 26:1, the union said.
The news came as new figures from business consultancy KPMG indicated that a third of colleges are actively seeking mergers, which could have implications for jobs.
The UCU based its calculations on a 25 per cent cut in funding to colleges, also factoring in that two-thirds of college expenditure is on salaries.
Although the UCU's figures describe the impact of an immediate 25 per cent reduction in spending, the Government's cuts timetable is phased over the next three to four years, up to 201415.
Sally Hunt, general secretary of the UCU, said: "The scale of the cuts facing further and higher education is staggering and will be a hammer blow to the country. We face the alarming prospect of class sizes rocketing and thousands of teachers in the dole queue at a time when they have never been more badly needed.
"The staff who survive the cull will have far less time to spend with students and will have far heavier workloads to deal with.
"The costs of educational underachievement in simple economic terms will be a creaking benefits system, a likely increase in anti-social behaviour and a huge loss in potential career earnings and tax revenues."
Shane Chowen, National Union of Students vice-president for further education, said: "The loss of lecturers means bigger classes, fewer classes and less choice for students. Lecturers are already under a lot of pressure, and adding to their workloads is not in the best interests of students."
Julian Gravatt, assistant chief executive of the Association of Colleges, said: "We don't accept these overly bleak predictions about projected job losses and would suggest that there are more sophisticated ways of interpreting the current difficult decisions facing colleges.
"Even when budgets are fixed, it is wrong to assume that a spending cut automatically means a job cut, because college governing bodies will have choices on pay, jobs, purchases and income generation - which they will make in consultation with their staff but with their main focus on the current and future needs of their students."
Paul Lawrence, KPMG's national director for education advisory services, said: "Our work in May shows that 34 out of 100 colleges were actively considering merger. Evidence now is that that figure will be higher.
"Fourteen out of the 34 were looking at merger from the point of view of acquisition, and the other 20 were saying that they did not think they could survive without merger."
Editorial, page 6.