Building work on hundreds of schools may have to be abandoned if the government is to fully implement its "free schools" policy, the Institute for Fiscal Studies (IFS) has said.
It said there was not enough money both to finish Labour's £55bn school renewal programme and fund free schools.
The government says free schools will improve the quality of education.
But shadow education secretary Ed Balls said cancelling Labour's building plans would "set back education".
The government's free schools policy encourages parents' groups, trusts, charities and other providers to set up their own schools.
'Has to give'
However, Building Schools for the Future was a central part of Labour's education reforms. It pledged to rebuild or renew every one of the 3,500 secondary schools in England by 2020.
The scheme started six years ago, with the initial projects targetted at less affluent areas.
Work is under way or finished in more than 400 schools, and another 1,000 are at various stages of planning.
Luke Sibieta, from the IFS, said it was "difficult to imagine" the government would be able to fund both projects and "something had to give somewhere".
"Either they have to scale back plans for free schools or reduce building schools for the future by a substantial degree," he said.
The government said it had not made any decisions yet on which Building Schools for the Future projects would go ahead.