The EU's nuclear safety watchdogs have deferred a decision on new safety test rules for Europe's 143 reactors.
Progress has been made by European Commission and ENSREG, a group of 27 national authorities for nuclear safety, on agreeing "stress tests", the European Commission said.
However the regulators are deadlocked over whether the stress tests should include testing resilience against terrorist attacks.
"Content is more important than timing," EU energy commissioner Guenther Oettinger said.
"The public expects credible stress tests covering a wide range of risks and safety issues - this is what we are working on."
The nuclear watchdogs are due to continue discussions in Prague next week.