The report said the cost could reach £900m – £800m more than the original estimate. It was due to be completed in 1997, but will not be ready until at least 2004 and is costing the European Union £155,000 a day.
The authors of the report, Vincent Van Quickenborn and Alain Destexhe, say it could "end up being one of the most expensive buildings in the world".
The project has been dogged by allegations of corruption and mismanagement. European Commission vice-president Neil Kinnock has launched a fraud investigation. Kinnock is said to be furious with the delays and has refused to pay extra costs.
Destexhe said part of the reason for the increase in cost is that there was insufficient expertise available to deal with asbestos when the project got under way in 1991.