Labour will need to find tens of millions of pounds of extra funding if it is to keep its pledge to save AS-levels.
Shadow education secretary Tristram Hunt said last month that he would block policies put forward by Michael Gove, the controversial former education secretary, that would abolish AS-levels.
But BuzzFeed has learned that sixth form colleges, which account for one-fifth of all A-level entries in England, are to scrap plans for their students to continue doing the recommended 4 AS-levels due to a lack of funding.
Jonathan Godfrey, principal of Hereford Sixth Form College, said that although the college "strongly supports a four-AS-level curriculum in sixth form", it will no longer be able to offer students that curriculum without more funding.
It is believed that a budget reduction equating to more than £500 per student would lead to a shortfall of tens of millions of pounds.
If a Labour government did not find the money, over 150,000 students at sixth form colleges would not have the opportunity to take four AS-level examinations, unlike their counterparts at independent schools. This would give the latter an advantage when applying to university.
Ian MacNaughton, principal of the Sixth Form College, Colchester, one of the largest in the country, said: "The funding is only there for three A-levels because the funding is only there for 15 hours a week. What the Labour party is saying is that we don't want the Gove changes.
"But you can't reverse the curriculum unless you reverse the funding as well."
Students typically choose four AS-levels when they are 16 and then drop one option for their final A-level examinations.
A Labour source said it plans to carry out a review of spending in the Department for Education. The source said: "This government should be held to account for the spending choices that it has made and the impact that these decisions are having on the 16–19 provision.
"Labour is undertaking a zero-based review of all departmental spending in the run up to next year's general election."