Students could combine school and college courses under the scheme
Teaching changes for 14 to 19 year olds in Wales should not proceed unless more financial details are made available, a key assembly committee has warned.
Under the proposals, schools and colleges would have to share resources to give pupils more study choices.
The finance committee said ministers had not demonstrated that their claims the system could be provided largely within existing budgets were realistic.
Ministers said they would "consider the findings and report back".
Schools and colleges already collaborate in this way in some parts of Wales, allowing pupils to chose some courses from schools and others from further education colleges.
The assembly government's plans, to be phased in over the next four years, would make all students in Wales entitled to this extra choice and enshrine this in law.
But the committee's report states that too many of the Welsh Assembly Government's financial calculations are based on "estimates and assumptions".
The document says "little if any work appears to have been done on ascertaining the true costs of implementing such a major new venture in education".
The committee concluded that it was "therefore unable to examine these calculations and has to conclude that it cannot reliably assess the impact of the proposed Learning and Skills (Wales) Measure".
The report added that the committee had "little alternative but to recommend that the stage one debate on the general principles of the measure is not brought forward until this information is available".
Only then, according to the committee, would AMs be able to "assess the financial impact of the measure".
The report goes further by warning that proceeding without such information would "seem irresponsible" given the "tight economic and budgetary situation the assembly is currently facing".
It also suggests that such a move would be "inconsistent" with the assembly government's aim of ensuring it gets value for money from its spending plans.
In response, a Welsh Assembly Government spokesperson said: "We will consider the findings and report back in due course."