LONDON — A few years ago, Britain agreed to let China take an ownership stake in its newest nuclear power plants, figuring Beijing had the nuclear know-how and the construction smarts to help replace the country’s aging power stations.

It was a warm moment in British-Chinese relations, a deal signed in 2015 during a choreographed visit to London by President Xi Jinping of China with the British prime minister at the time, David Cameron.

Six years later, Britain is having second thoughts.

Financing for a planned power station facing the North Sea, estimated at $28 billion and necessary to ensure a steady stream of electricity for decades, is unexpectedly in doubt. Part of the problem: attracting investors to a project one-fifth owned by China.

Xi’s authoritarian ambitions and human rights record have chilled relations with Western nations, forcing a broad reconsideration of a range of economic dealings with the world’s second-largest economy.

The 2015 nuclear agreement even calls for letting China be majority owner of a proposed plant of its own design, at a site about 50 miles from London. Although that project is going through regulatory channels, it is expected to face strong opposition from lawmakers.

“We cannot allow the technological heart of our power system to be exposed to the risk of disruption by states that do not share our values,” said Tom Tugendhat, a member of the Conservative Party, led by Prime Minister Boris Johnson, and chairman of the foreign affairs committee in Parliament.

Evidence of the risks involved was buried in financial results published last week by Electricite de France, a French utility company that owns and operates Britain’s eight operating nuclear power stations. The company is halfway through building Britain’s first new station since the 1990s, at Hinkley Point in southwest England, a project one-third owned by China General Nuclear, China’s state-owned nuclear company.

EDF, in its quarterly results, urged the British government to pass legislation soon enabling a new, less risky financial and regulatory arrangement before the company embarks on the North Sea project, near a fishing village called Sizewell.

Failure to obtain these changes, the company said, could lead it to “not to make an investment decision” — in other words, walk away from the project.

EDF, which is majority owned by the French government, says it can’t afford to pay the project’s costs upfront and wants to reduce its 80% stake to a minority holding to make room for other investors.

The critical question, though, is whether the presence of China General Nuclear might give financial institutions pause, especially those from the United States.

In 2019, the company was placed on a U.S. government blacklist — which restricts U.S. companies from doing business with it — for engaging in efforts to acquire advanced U.S. nuclear technology for military purposes. In 2016, a U.S. nuclear engineer was sentenced to two years in prison for helping the company develop nuclear materials.

China General Nuclear declined to comment.

Ultimately, the government will decide the fate of Britain’s nuclear program; one option said to be on the table is the British government’s buying China’s stake in the Sizewell project.

In principle, the government wants at least one more power station after Hinkley Point to help meet its low-carbon targets. The Sizewell plant would pump out enough power for millions of homes for decades. Building a plant would also create thousands of jobs and provide money for British suppliers.