Hitachi said "no formal decision" has been made over the future of a UK nuclear plant following a report it would halt construction.
The Nikkei Asian Review reported the firm's board would be likely to decide to suspend all work on the Wylfa Newydd plant next week.
In December, the firm said it would do its utmost to ensure the nuclear power facility went ahead.
A statement from the Welsh government described the report as "worrying".
"This is a major project with significant economic benefits to Wales and rest of the country," the statement said.
"We will continue to monitor the situation very carefully and press the UK Government do to everything it can to help bring this project to Anglesey."
Shares in the Japanese company jumped 8% after the report.
Speculation has been mounting that Hitachi will scrap the project, due to potential increases in construction costs.
On Friday, the firm said suspension of the project remained an option.
"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.
The UK government had been in formal negotiations with the Japanese firm over the project since June.
Hitachi announced its agreement to take over the Horizon project in 2012.
The plant would aim to have a generating capacity of 2900 MW by the mid 2020s and have a 60-year operational life.
The original Wylfa nuclear plant near Cemaes closed in 2015 after more than 40 years' service.