Anglesey Island, where the plant, called Wylfa Newydd, was to be built, will most likely feel the harshest impact. Llinos Medi, leader of the Anglesey Council, the local government authority on the island, said in a statement that she was concerned about the suspension’s “immediate impact on local men and women whose employment is at risk as a consequence of this suspension.”

For Hitachi, though, the announcement could mark the end of a long and expensive saga. The company acquired the Horizon sites from two German utilities in 2012 for £697 million, or about $900 million, and wound up spending around £2 billion in total on design approvals, staff and other matters. It has been hiring apprentices, who have been training at a technical college on the island and going to Spain and Japan for work experience. At times in recent months more than 100 archaeologists were on the site, excavating and recording ancient structures that the construction would have destroyed.

Hitachi hoped Britain would prove to be an international showcase for its reactor designs. Ultimately, the company lost patience with the high level of spending required to land such a project there.

Hitachi had sought to arrive at a financial arrangement that would attract long-term investors like pension funds to the project and reduce its own exposure. But the offers of support from both the British and the Japanese sides were not enough.

In an interview on Thursday, Duncan Hawthorne, Horizon’s chief executive, said the Wales site was very attractive and had a supportive community, but that alone, he added, is not enough to attract outside investment.

Investors, Mr. Hawthorne said, were put off by the eight- to 10-year wait for a return, as well as the risks of cost overruns and other issues. With the project gaining momentum and spending rising, “it became too much for Hitachi to keep going,” he said.

Mr. Hawthorne said Hitachi would prefer to return to Britain as a supplier rather than a developer taking on large upfront risks.