Nippon Steel and U.S. Steel are suing the U.S. government to block the takeover, claiming President Joe Biden is “ignoring the rule of law to win favor with unions.”

The companies also claim that President Joe Biden, who is entering his final weeks in office, is blocking Nippon Steel from acquiring U.S. Steel to pursue his own political agenda.

Rejected the proposed deal on FridayPresident Biden said a strong domestic steel industry is critical to national security and resilient supply chains, including the automotive and defense industries.

If the $14.9bn (£11.8bn) deal goes ahead it will create one of the world’s largest steel companies outside China.

The acquisition has been pending since it was first announced in December 2023.

In a lawsuit, Nippon Steel and U.S. Steel asked the court to put on hold the review process of the Committee on Foreign Investment in the United States, which has the authority to review foreign acquisitions of U.S. companies, saying it “failed to perform its duties well.” Faith, National Security-Focused Regulatory Review Process.”

Additionally, the companies are suing United Steelworkers President David McCall and rival Cleveland-Cliffs Steel CEO Lourenco Goncalves for “their Unlawful and coordinated actions aimed at stopping the transaction.”

On Friday, the United Steelworkers union said there was “no doubt” blocking the takeover was “the right move for our members and national security.”

The union accuses Japanese steel companies of sabotaging the U.S. steel industry for decades, including by dumping their products on the U.S. market.

Nippon Steel and U.S. Steel said they had “engaged in good faith with all parties” to demonstrate how the deal would “enhance, rather than threaten, U.S. national security” and strengthen the U.S. domestic steel industry “against threats from China.”

They also reiterated Japan’s readiness to invest $2.7 billion in Pittsburgh-based U.S. Steel.

President-elect Donald Trump has also said he would block the deal. “Why would they sell U.S. Steel now when tariffs will make U.S. Steel a more profitable and valuable company,” he posted on Truth Social on Monday.

Earlier on Monday, Japanese Prime Minister Shigeru Ishiba expressed concern about the U.S. decision to block the takeover and the impact it might have on trade relations between the two G7 countries.

“Unfortunately, we do hear concerns from Japanese industry about future investments between Japan and the United States. We must treat this issue as a serious one,” Mr. Ishiba said.

He added that while it would be inappropriate for the government to comment on individual companies, “we must insist on explaining why there are safety concerns or future discussions will not progress.”