The existence of the document, written by Robert Galvin, a senior EUofficial who is the parliament's chief internal auditor, was first disclosed by The Daily Telegraph in 2008.
Despite public controversy across Europe over misuse of generous allowances paid to MEPs, parliament officials have fought tooth and nail to keep the Galvin report, number 06/02, secret.
During an internal investigation, Mr Galvin discovered large scale abuse of £185 million in MEP staffing allowances and general expenditure expenses, paid without receipts.
Parliament lawyers argued that publicising the report "could be used to derail" decision-making in the EU assembly.
"The use members make of the allowances available to them is a sensitive matter followed with great interest by the media," said a legal submission.