Federating schools only helps those with poor results and will have little impact on performance if it is done only to save money, ministers have been warned in research commissioned by one of the Government's own quangos.
Schools Secretary Ed Balls claimed #163;2 billion could be raised by joining primaries and secondaries together and employing fewer headteachers.
But now new research says the education system is not ready for more federations and there are "challenges" to resolve before they should become commonplace.
The study, commissioned by the National College for Leadership of Schools and Children's Services, the organisation charged with developing new heads, said there is currently no way of holding federations to account. Other barriers include the difficulties of achieving collaboration between "challenging" schools, and working within country-wide schemes such as National Challenge.
The report's authors, from Oxford and Manchester universities, said that, overall, federations can have a "positive impact" on children's attainment, but only when the aim is to raise standards by linking higher and lower-performing schools.
The academics studied the schools for two years, from 2006, and compared them with those that had not joined together.
"Overall, the main differences in performance between federation and comparator schools appear in performance federations.
"The evidence for cross-phase federations is mixed, while few or no significant differences were found for the other types."