Hundreds of problems have been found at European nuclear plants that would cost 25bn euros (£20bn) to fix, says a leaked draft report.
The report, commissioned after Japan's Fukushima nuclear disaster, aimed to see how Europe's nuclear power stations would cope during extreme emergencies.
The final report is to be published on Thursday. The draft says nearly all the EU's 143 nuclear plants need improving. Anti-nuclear groups say the report's warnings do not go far enough.
For its part, the regulatory body for European nuclear safety has urged the Commission not to use language that could undermine public confidence, says the BBC's Chris Morris in Brussels.
The report - the wording of which could change before Thursday's final version is published - points out that in the EU, 47 nuclear power plants with 111 reactors have more than 100,000 inhabitants living within a circle of 30km.
"On the basis of the stress test results practically all [nuclear plants] need to undergo safety improvements," says the leaked draft. "Hundreds of technical upgrade measures have already been identified.
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