Balls: I have evidence of cash for admissions claims
The Children, Schools and Families Secretary told The TES he was also confident the research would back up his decision to accuse a significant minority of schools of breaking the admissions code before he had checked his information.
Mr Balls' comments last week prompted criticism from some heads and accusations from Conservatives that he was trying to distract attention from national figures that revealed that nearly one in five pupils was not admitted to their first choice of school.
The Statistics Commission watchdog wrote to Mr Balls's civil servants to advise that official statistics should not be associated with other ministerial statements.
Mr Balls said: "The idea that we were attempting to conceal one piece of information with another, or to give one a greater priority - I don't think that was in our minds."
Desk research by Mr Balls's officials revealed apparent breaches of the code by schools in three authorities. But subsequent TES enquiries suggested that the practice of requesting financial contributions was restricted to seven primaries in Barnet in north London.
Michael Gove, the shadow schools secretary, read out in Parliament this week two quotes from heads who were angry at the way the information had been released.
Mr Balls said a parent had emailed him to say a school had asked for fees of pound;37.50 a month in its prospectus. It was one of several new cases in other authorities that he said had emerged following last week's publicity. But only those in Northamptonshire, Manchester and Barnet are being investigated by his department.
Mr Balls said he was pleased that the Church of England, Catholic education service and Jewish Board of Deputies had all said they would work with the Government to ensure that all schools followed the admissions code.
All local authorities are now being given until the end of June to submit a full set of 2009 school admission arrangements and opinions on their legality to the chief schools adjudicator, who will report back with an interim verdict the following month and issue a final report by September 1.