A decision on whether to give a stalled £13bn nuclear power project planning permission has been deferred.
Business Secretary Andrea Leadsom wants more information on environmental and other impacts for Wylfa Newydd on Anglesey.
She had been widely expected to back the proposals, granting what is known as a development consent order (DCO).
Hitachi shelved the scheme, the biggest energy project ever proposed in Wales, over funding issues.
Developers Horizon Nuclear Power said it was disappointed the decision had not been made on Wednesday.
Earlier it had said the decision would "heavily influence" how the project progressed.
Ms Leadsom has now given a deadline by the end of the year - and invited comments from Natural Resources Wales, the Office for Nuclear Regulation, Anglesey council and other bodies.
She wants more assurances on various aspects - from biodiversity, visual impact, flooding and construction noise - and any risk to the Sandwich tern, which has a colony nearby.
Planning inspectors spent six months examining the plans and considering their impact on local people and the environment, before making a recommendation to the UK government.
About 9,000 workers were expected to be involved in building the two nuclear reactors, which were due to be operational by the mid-2020s, supplying up to five million homes for 60 years.
But Japanese energy giant Hitachi put plans on hold after failing to reach a deal with the UK government over the price it would be paid for power from the site.
Since then ministers have been consulting on new ways to fund big, expensive nuclear projects.
Opponents of nuclear power have called on Ms Leadsom to dismiss the planning application and focus on renewable sources of electricity.
Dylan Morgan, of People Against Wylfa B, said it was "obvious the developers are keen to get planning permission in order to try and sell the site".
"But that's easier said than done at the moment given the pretty perilous state of the global nuclear industry and the hopeless economics," he said.
Horizon Nuclear Power - a subsidiary of Hitachi - said it would now review the business secretary's comments in detail and work towards addressing the points she raised.