After last year's Fukushima nuclear catastrophe in Japan, the Commission had called for states to complete their probes – also known as ‘stress tests’ – by around the middle of this year.
But Oettinger said on Thursday (26 April) that that some reactors still needed to be assessed and it was more important to be thorough than fast. A final report is not now expected until the autumn.
So far, 38 nuclear reactors have been visited out of the total of 147 in the European Union. The commissioner did not specify how many more needed to be inspected, but said it was important to have "a complete overview", taking into account all types of reactor.
After that, the Commission might agree to legislative proposals to improve nuclear safety.
"We will bring our proposals of what should be harmonised after the stress test report. Maybe that will be in December, or the first few months of 2013," he told reporters.
The voluntary stress tests are meant to establish whether nuclear plants can withstand natural disasters, aircraft crashes, management failures and whether adequate systems are in place to deal with power disruptions.
"EU citizens have the right to know and understand how safe the nuclear power plants are they live close to. Soundness is more important than timing," Oettinger said in a separate statement.
One lesson of the Fukushima tragedy was that two natural disasters could hit at the same time and knock out the electrical supply system of a plant completely, preventing it from being cooled down.
The EU embarked on the tests among the 14 member states that operate nuclear plants as a first stage of its response.
EU member Lithuania, which is decommissioning its nuclear units, is also taking part and from outside the 27-member bloc, Switzerland and Ukraine joined the test regime.
The 38 EU reactors that have been inspected so far include units at Heysham in Britain, Tricastin in France, Forsmark in Sweden, Grafenrheinfeld in Germany and Ignalina in Lithuania.
Although the stress tests have not been completed, a report from the European Nuclear Safety Regulators Group (ENSREG) will be presented to ministers in June. It has been endorsed by EU states, with the exception of Austria, which is a vocal opponent of nuclear power. It banned atomic plants in 1974.
"Our demand is clear, European nuclear plants have to retrofit or shut down," the Austria Press Agency quoted Austrian Environment Minister Nikolaus Berlakovich as saying.
Green groups have also questioned whether the EU is doing enough and Germany said last year that it would phase out all its atomic plants by 2022, while Italy voted to ban nuclear power for decades.
Andrej Stritar, chairman of ENSREG, said the review concluded that all countries had taken "significant steps" to improve the safety of their plants.