Almost half of England's further education colleges have seen a decline in student numbers - with the drop blamed on the scrapping of the Education Maintenance Allowance (EMA).
An Association of Colleges survey shows 49% have fewer students than last year.
Financial pressures, such as the loss of the EMA allowance and the cost of transport, are named as the reason.
The Department for Education said financial support was now targeted at students "who need it most".
Fiona McMillan, president of the Association of Colleges and principal of Bridgwater College, says that it is particularly the poorest students, with the lowest skill levels, who are not enrolling.
These youngsters are the most vulnerable to the loss of financial support, she says, with practical barriers such as the cost of bus fares being enough to deter applicants.
"We know of students who cannot afford to get to college," says Ms McMillan.
The survey has been trying to gauge the impact of the removal of the EMA, which had provided up to £30 per week in means tested support for students.
It found that the scrapping of the allowance was cited by colleges as the biggest single reason for a fall in numbers.
Ms McMillan says that the loss of the EMA does seem to have put off the poorest students.
"For people with very little, any extra cost is too much," she said.
What worries her now is the lack of an effective support system to replace EMA.
"While the loss of the EMA might not have been a complete surprise, what has been a surprise has been the lack of an adequate replacement," she says.
In her own college, she says that the EMA could provide students with about £1,000 per year - but now there is only £152 per year available for students who would have qualified for EMA.
The survey found that many colleges were topping up support for poor students from their own budgets.
She warns that the type of youngsters who are now not enrolling are going to be "hanging around and not doing very much".
But the survey, which included further education colleges, sixth form colleges and specialist colleges, also reveals a mixed picture in enrolments.
While 49% reported falling student numbers, 42% had an increase, with the remainder remaining stable. The biggest increases were at the upper end of the skill level, for A-level courses and their vocational equivalents.
There is also a demographic factor - with 40,000 fewer people in this age group than the previous year.
The Department for Education challenged the survey - saying that the evidence did not provide clear evidence of a decline in numbers.
"We don't think the AoC survey is particularly robust. There is a drop in overall student numbers of 40,000 this year - and this survey shows a drop of 600 overall," said a DFE spokesman.
"Only half colleges choose to respond and even of them, the majority showed the enrolment numbers were steady or risen and over a quarter had risen between 5% and 10%.
"Record numbers of 16 and 17-year-olds are already in education or training. We are increasing that further with more than 1.5 million places available from this term - with every student guaranteed a suitable place in sixth-form, college or work-based training."
The DFE also defended the more closely targeted £180m bursary scheme, which replaces the EMA, which had cost £560m per year. It says that under the new scheme students who would have been eligible for free school meals could receive up to £800 per year.
But shadow skills minister Gordon Marsden said: "Today's alarming figures on College enrolments released by the AoC show clearly the cumulative impact of this Tory-led Government's policy to scrap EMA combined with their failures over the All-Age Careers Service.
"These findings are underlined by today's devastating figures on Youth Unemployment which show their failure to have a strategy for growth which will start remedying this."
James Mills, head of the Save EMA campaign, said: "These figures suggest that ending EMA has meant that class is being brought back into the classroom, as these figures will mean that not only those poorer students staying on have a bigger mountain to climb but many are prevented from even leaving home."
"In a week when unemployment is at a 17-year high and we see child poverty is set to rise, we now have more proof that children born into poverty are going to stay in poverty. Because they will be priced out of education and priced into unemployment."