Adult education in England "will not exist by 2020" if government cuts continue, colleges have warned.
The Association of Colleges says 190,000 adult education places will go next year, as funding is cut by 24%.
Courses for health, public services and care, and information and communication technology (ICT) could be hardest hit.
The government concedes total funding has been reduced but says it has prioritised apprenticeships and subjects including English and maths.
The Association of Colleges (AoC) represents 336 institutions in England, including general further education, sixth-form, tertiary and land-based colleges.
'Swathe of cuts'
It says: "Adult education and training in England will not exist by 2020 if the government continues with its swathe of cuts."
The number of adult students participating in Level 3 courses - including BTecs and NVQs - fell by almost 18% between 2013 and 2014, it says.
And if funding continues to be cut at the current rate, "there will no longer be an adult education system remaining to support students aged 19 and over".
Martin Doel, the AoC's chief executive, said the cuts "could mean an end to the vital courses that provide skilled employees for the workforce, such as nurses and social care workers".
"The potential loss of provision threatens the future prospects of the millions of people who may need to retrain as they continue to work beyond retirement age, as well as unemployed people who need support to train for a new role.
"Adult education and training in England is too important to be lost, to both individuals and the wider economy."
'Alarming, but realistic'
The Department for Business, Innovation and Skills (BIS) acknowledged budget cuts had been made, but said the most important areas had been protected.
"We fully recognise the important role further education plays in getting people the skills they need to get on," a BIS official said.
The official added that more than £3.9bn had been committed to adult learning and further education in 2015-6, including £770m for apprenticeship funding.
"While total funding has been reduced, priority has been given the areas where the most impact can be made - apprenticeships, traineeships and support with English and maths.
"Many colleges and training organisations have responded well to the need to find other income streams for skills provision, and it is this approach that will help them succeed."
Prof Ewart Keep, from Oxford University's department of education, says the AoC's analysis is "alarming, but realistic".
"The latest reductions raise the prospect of provision reaching a tipping point, from which subsequent recovery could be very difficult," he said.
"Cumulative cuts of this magnitude are extremely difficult to absorb, and mean that those colleges and other providers who have a strong focus on adult learners may either go out of business or be forced to re-focus their attention on younger, pre-19 students."
The National Institute of Adult Continuing Education (NIACE) has also supported the AoC's analysis.
NIACE's chief executive, David Hughes, said England was facing "a skills crisis".
"We should all be telling young people to get their state-funded learning in as quickly as they can," he said.
"Once they hit 21 there won't be any support left.
"That is not a great scenario for a society in which people are living longer and wanting to contribute to society and work longer too."