EU safety tests on nuclear plants should be completed by the middle of the year to ensure they are sufficiently thorough.
Ahead of the anniversary of the Fukushima nuclear disaster on March 11, EU Energy Commissioner Guenther Oettinger said stress tests that had been expected to be finished by the end of June would be completed "not later than summer".
"Thoroughness is more important than delivering quick results. Our multinational teams still need some time to finish the tests. Not later than summer we will publish the results," he said in a statement.
He said the tests were strict and objective and included testing on whether nuclear plants could withstand natural disasters, aircraft crashes and management failures.
The EU embarked on the tests in response to the Fukushima tragedy.
Green groups and politicians have questioned whether the EU response is strong enough, while Germany last year said it would phase out all its atomic plants by 2022 and Italy voted to ban nuclear power for decades.
Oettinger has said the stress tests offer the potential to reassure on nuclear safety well into the future and that cooperation across the bloc on a very sensitive issue was significant progress.
The stress tests are voluntary, but all 14 member states that operate nuclear plants are taking part.
"In the past it would have been unthinkable that multinational expert teams would be granted access to nuclear power plants," he said.
"Nuclear safety was and is primarily a national competence. The fact that multinational teams conduct the tests increases objectivity and adds a true European dimension to the tests."