Government plans to turn all schools into academies face a £1.1 billion funding shortfall, according to Labour, who said the costly scheme could harm the education system.
In his recent Budget, Chancellor George Osborne announced plans to turn every state school into an academy by 2022, although Labour warned just £140 million was set aside for the conversions.
The Government dismissed the suggestion there will be a shortfall as "completely untrue".
Figures obtained by Labour from a parliamentary question suggest each transformation from school to academy costs about £66,000 on average. They said councils would have to cover a further £12,300 in costs, such as legal fees, per school.
The Government intends to turn 16,800 schools into academies by the end of this parliament. Those that have not converted must have plans in place to do so by 2022.
Under Labour's calculations, the cost to the Department for Education and local authorities would be more than £1.3 billion.
Shadow education secretary Lucy Powell said the added cost came at a time when schools were facing "huge challenges", including reduced budgets.
"This costly reorganisation of our schools is an unnecessary and unfounded distraction, which could harm standards in our schools," she said.
A Department for Education spokeswoman said funding earmarked in November's Spending Review and the March Budget would be enough to support a "high-quality, fully-academised school system".
"We have over £500 million available in this parliament to build capacity in the system - including recruiting excellent sponsors and encouraging the development of strong multi-academy trusts," they added.