Serious doubts have been raised over whether any new academies will be open by the Government's target date of September 1.
The first heads to sign up to the flagship policy say they are unlikely to be ready, a blow for Education Secretary Michael Gove, who had said "significant" numbers would be ready for the start of next term.
It has also emerged that as few as 50 schools may have been approved for the switch to academy status. The Association of School and College Leaders, which supplied the figures, said a further 50 had been told they needed to do further work on their applications.
Mr Gove began to send letters to schools that have been approved for academy status more than two weeks ago, before the legislation was passed in Parliament this week.
But governors have to complete a process that includes setting up limited companies, appointing directors, setting up payroll services and leasing land and buildings.
They also have to register with exam boards and sign new contracts with those providing services. Mr Gove then has to approve their "funding agreement", which is the final stage of the process.
Department for Education (DfE) officials have said they are "relaxed" about when schools become academies, but schools minister Nick Gibb said in Parliament this week that ministers "are in a hurry to have them by September".
Despite this, he admitted to MPs, the process of signing funding agreements will "go on through the autumn".
Shadow schools minister Vernon Coaker said the Government had refused to put a figure on the number of schools expected to become academies by September. "That now seems to have gone down to almost nought," he said, describing it as a "huge retreat".
The application from Oldfield School in Bath was one of the first to go to the Education Secretary for approval. But head Kim Starling said: "We have to be realistic, but it is disappointing that we won't be open as an academy by September 1.
"We've started proceedings for the transfer of staff to the new school, but there is a lot of work on the financial side to do and we can't do that during the summer holidays when people are off."
Staff at Uffculme School in Devon were told that their academy application had been approved two weeks ago.
But Richard Carman, its chair of governors, said: "Even supporters of the legislation would say it was hastily put together. We want to make this happen, but anything could happen in the next few weeks."
Martin Freeman, head of pay at the Association of Teachers and Lecturers, said: "Most of the schools that have applied will miss the target date. I predict more are likely to get academy status by mid-September or October.
"It's very unwise for schools to be aiming for academy status by September 1. It's more macho behaviour than sensible leadership."
Chris Keates, general secretary of teaching union the NASUWT, said schools rushing to open in September could be vulnerable to legal challenges and employment tribunal claims.
"The indecent haste with which some outstanding schools have rushed to apply for, and secure, academy status means that a number appear to have cut corners in terms of consultation with staff and unions," she said.