A pair of former British politicians say President Biden’s opposition to the $14.9 billion sale of U.S. Steel to a Japanese company will hurt global relations, and accuse him of blocking the deal solely to score political points.

Sir Gavin Williamson, the former U.K. Secretary for Defense, and James Wharton, the Baron Wharton of Yarm, a former member of the British Parliament, recently sent letters to the Department of Treasury, Department of Commerce and International Trade Administration.

The letter also has been circulating on Capitol Hill.

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It said that Mr. Biden’s opposition could result in the politicization of the Committee on Foreign Investment in the United States, the federal committee that reviews foreign investment in U.S. companies. They say that could “have far-reaching consequences” on America’s relationships with international allies.

“Economic isolationism, driven by political calculations threatens not only America’s global standing but also the health of its economy,” Mr. Williamson and Mr. Wharton wrote. “Historically, economic isolation has led to decreased supply, higher prices and stunted growth. Instead of leveraging CIFUS to advance political agendas, we should maintain an open and predictable investment environment to attract and retain foreign investment.”

The pair said the decision to block Nippon Steel’s purchase of U.S. Steel “threatens to undermine America’s relationships with key allies and disrupt the flow of vital foreign investment.”

The White House and the Treasury Department did not immediately respond to a request for comment.

Mr. Biden and Vice President Kamala Harris have been steadfast that U.S. Steel should remain domestically owned. Former President Donald Trump has also pledged to block the deal if elected.

“I feel very strongly that U.S. Steel needs to remain a U.S. company and that people working there need to be American workers,” Ms. Harris, the Democratic presidential nominee, said Tuesday. “I think that is also why I’m proud, and I do have the support of the steelworkers unions.”

U.S. Steel warned last month that it may have to move its headquarters from downtown Pittsburgh and slash thousands of union jobs if the sale to Nippon Steel fails.

U.S. Steel said that without the deal, it would have to pivot away from its blast furnace facilities, resulting in the loss of union jobs. The company said that moving its headquarters out of Pittsburgh would cause a loss of $3.6 billion in economic activity in the region.

Pennsylvania’s two Democratic senators, John Fetterman and Bob Casey, have pushed back on the company’s claims. Mr. Casey said the pleas of economic hardship if the deal isn’t finalized shows “a deep disrespect” for workers.

In a statement last month, Mr. Casey accused U.S. Steel’s corporate leadership of “refusing to invest in Southwestern Pennsylvania,” and their threats are another example of putting “livelihoods at stake if he does not get what he wants.”