The National Union of Students says nearly 400,000 out of Britain's 3. 2 million students are affected by the 16-hour rule, which limits the number of hours people can spend at college without losing benefit.
Last year when the benefit rules were changed the then employment minister Ann Widdecombe said the figure was only 80,000.
NUS president Douglas Trainer said: "These figures show the devastating state of finance in the further education sector.
"The colleges are reliant on the fees from these students, the government has woefully under-estimated the numbers and the number of unemployed people using study to improve their job chances is huge and growing.
"We want a commitment from any future government to address the issue urgently.
"Access to study is absolutely crucial for the unemployed who want to better their chances of finding future employment, but we need to make sure the source of expenditure for this is clear and safe.
"This group of students and the finances of further education colleges will remain vulnerable until this paradox is resolved, especially as the issue of the funding for 18- to 25-year-olds on the dole is currently a political ping-pong ball.
"We want all the political parties to face up to the reality of the situation as confirmed by these figures and to end the uncertainty and confusion affecting nearly 400,000 claimants."