The prime minister has been accused of focusing on Brexit rather than fighting to ensure a new nuclear power station planned for Anglesey goes ahead.
Reports emerged on Friday that Hitachi’s board would be likely to decide to suspend all work on its Wylfa Newydd plant next week.
Unions and politicians criticised Theresa May for not discussing the issue with her Japanese counterpart.
The UK government said negotiations with Hitachi were ongoing.
Speculation has been mounting that the Japanese company will scrap the £20bn Horizon nuclear power plant project due to potential increases in construction costs.
Anglesey MP Albert Owen said he wanted to know from ministers “what is going wrong here and why these questions and fears are being heard as regards the future of the site?”
He added: “The government has been concentrating too much on Brexit.”
Unite the union said it was “astonished” that the situation was not discussed by Theresa May and Japan’s Shinzo Abe when they met on Thursday.
“This was an abdication of responsibility by Theresa May,” said the union’s national officer for energy, Peter McIntosh.
“The project appears to be teetering on the brink which would be a really big setback for the UK’s future energy policy.”
He added that the UK government needed to take action to ensure the Wylfa project moves forward, adding that if the plant was not built it would “have a devastating impact on the Welsh economy and on the UK’s ability to meet its climate change obligations”.
Justin Bowden, national secretary of the GMB union, said the UK government needed to offer to help fund the plant to ensure the UK’s energy needs were met.
“Government must act and step in now, picking up the reins and taking whatever funding stake is necessary, to ensure Wylfa goes ahead on time,” he added.
The Welsh Government has described the report in the Nikkei Asian Review about the Wylfa plant being scrapped as “worrying”.
The new nuclear plant would aim to have a generating capacity of 2900 MW by the mid 2020s and have a 60-year operational life.
On Friday, the firm said suspension of the project remained an option.
But it said the latest news report in Japan were not “based on Hitachi’s decision or disclosed information”.
“No formal decision has been made in this regard currently, while Hitachi has been assessing the Horizon project including its potential suspension and related financial impacts in terms of economic rationality as a private company,” it said in a statement.
A spokesman for UK government’s Department for Business, Energy and Industrial Strategy, said: “Negotiations with Hitachi on agreeing a deal that provides value for money for consumers and taxpayers on the Wylfa project are ongoing.
“They are commercially sensitive and we do not comment on speculation.”