The government's education technology agency has signed a secrecy clause that prevents it from disclosing how much money the world's wealthiest software company is making from British schools.
Microsoft has released details of its schools software sales to Becta, the body that advises schools on technology. But the figures cannot be made public because of a confidentiality agreement.
Becta has already raised concerns that Microsoft is locking schools into using its products with licences that impose "unaffordable" buy-out fees.
The agency found that more than 70 per cent of schools and colleges were unaware of the size of these fees and called for the company to reduce them "urgently".
When an MP requested further information, the agency was unable to help.
Brooks Newmark, a Conservative member of the Commons science and technology committee, tabled a parliamentary question asking how much schools had spent on Microsoft products in each of the past three financial years.
Jim Knight, the schools minister, said that such information was not collected from schools.
"Confidential information has been made available to Becta by Microsoft in respect of Microsoft's licensing sales to schools," he said. "This information is the subject of legally binding non-disclosure arrangements."
It is estimated that more than 95 per cent of British schools use Microsoft software packages such as Windows and Office.
Becta has advised schools to hold off on buying the company's new products, Vista and Office 2007, because costs were significant and the benefits still unclear.
Bob Fenton, Becta spokesman defended the agency's confidentiality agreement, saying it was important for it to maintain a good relationship with the company.
Microsoft said it was unable to comment.