The Education Secretary has been challenged to provide evidence supporting the Government's plans for sweeping school reforms.
Nicky Morgan has come under fire from the National Union of Teachers (NUT) over the Education Excellence Everywhere White Paper, which included proposals to turn every state primary school in England into an academy by 2020.
NUT deputy general secretary Kevin Courtney sent an open letter to Ms Morgan on Wednesday, outlining concerns that the Department for Education's plans are founded on scant evidence.
In the letter, he wrote: "You propose the forcible academisation of 17,000 primary schools. However, and astonishing for such a far-reaching proposal, there is no evidence section in the White Paper to support this belief.
"Can you explain why you have not produced evidence which shows academy schools doing better than comparable local authority schools?"
In an address to journalists at the NUT's spring conference in Brighton, Mr Courtney said the department was misleading people by comparing the progress of sponsored academies - failing state schools forced to become an academy as part of a government intervention strategy - with successful local authority control schools.
He said: "They (the department) have given us absolutely tortured statistics to justify this huge proposal. They say the results show primary-sponsored academies opened for two years have improved their results by 10%, more than double the rate of improvement in local authority-maintained schools.
"That is statistically meaningless. They are not comparing like for like.
"It is like me as a not very good jogger - if I trained for another four hours a week I would get quite a lot faster, and if (Olympic gold winner) Mo Farah trained for another four hours a week he wouldn't improve at all because he's already going as fast as you can go.
"They are comparing schools that are already doing well and saying they have not improved much in the last two years.
"They are already as far as you can get - of course they haven't improved much. Whereas schools that aren't doing well can improve."
He said the real comparison should be between struggling academies and struggling local authority-maintained schools.
Continuing the running analogy, he said: "When you look at those statistics, you find the local authority jogger does better than the academy jogger."
In its White Paper, the Department for Education said it believed that "the fastest and most sustainable way for schools to improve is for government to trust this country's most effective education leaders, giving them freedom and power, and holding them to account for unapologetically high standards for every child, measured rigorously and fairly".