A report by government experts said the building projects could exceed forecasts by half a billion euros on a total planned budget of 6.8 billion euros.
The watchdogs also warned of delays in completing the Olympic villages for athletes and for media which will be built alongside the River Seine and urged reduction in the scope of the projects and the size of the joint site to keep cost overruns below 200 million euros.
The report by the French inspectors general of finance, sport and environment is the official version of a document that had already been leaked. It is primarily concerned with ensuring the public contribution to the costs do not exceed the 1.5 billion euros earmarked by the French government
The construction budget for the Games had already risen to 3.0 billion euros.
The planned Olympic villages will spread across three communities north of Paris. The athletes village is intended to accommodate 17,000 competitors before becoming 3,000 housing units.
The inspectors pointed out that the site has also been earmarked as a workshop for construction of a metro extension, contains archaeological remains, a school and a residence for migrant workers.
The report said the plan was "to complete in six years a project that would take 10 years" under normal circumstances.
The report was also uneasy about a planned aquatic centre for swimming and diving next to the existing Stade de France in Saint-Denis, just north of Paris, which will host the athletics.
The inspectors point, first, to a gap between the budgeted cost of 108 million euros and their calculation that the actual bill "could exceed 260 million euros, not including tax."
They also warn that the cost of operating the pool will be 500,000 euros a year and that "no-one is paying for that".
Finally, the report prepared warns that the Olympics could also cause delays for other infrastructure projects around Paris, notably the Grand Paris Express network of metro line extensions.
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