The report by finance, sports and infrastructure inspectors called for Olympic village plans to be revised to take account of remnants of archaeological value at the site, a residency for migrants and a high school.
It said plans for a media village should be scaled down and it warned that the swimming centre and surrounding developments were at risk of running as much as 170 million euros ($210 million) over budget.
"At this point the total potential risks identified by the cases under consideration stand at 500 million euros," the report said, adding the figure could be kept to 200 million if its recommendations were followed.
"Nonetheless, the main risk is that indispensable structures are not delivered in time for the Olympic and Paralympic Games," it said.
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The overall budget for hosting the games was initially set at 6.8 billion euros, with an investment of public money of 1.5 billion euros.
Separate reports from the Senate and the public audit office have already warned that a massive expansion of the Paris metro network may run billions over budget and not be ready in time for the Olympics.
(Reporting by Leigh Thomas; Editing by Geert De Clercq and Richard Balmforth)
