(Reuters) -U.S. President Joe Biden has decided to officially block Nippon Steel's proposed purchase of U.S. Steel, a person familiar with the decision said on Friday.

Biden, President-elect Donald Trump and a politically-influential labour union had voiced opposition to Nippon Steel's acquisition of U.S. Steel, which helped build the Empire State Building and arm allied forces in World War Two.

World Steel Association data showed the estimated $15 billion merger deal would have created the world's third-largest steelmaker after China's Baowu Steel Group and Luxembourg-based ArcelorMittal.

U.S. Steel's shares had tumbled in recent weeks following a Bloomberg report that Biden planned to kill the merger.

Here is a look at the latest developments:

APPROVAL

The Committee on Foreign Investment in the United States, a government panel that assesses inbound foreign investment for national security risk, had been reviewing the transaction for months.

In late August, it notified the two companies of associated risk, Reuters reported, and days later sources said Biden was poised to block the deal.

But the panel opted to extend deliberations, pushing the decision back to after the Nov. 5 presidential election, Reuters reported in September.

Nippon Steel had repeatedly said it was confident of closing the deal by the end of 2024.

TRUMP'S STANCE

Trump had repeatedly vowed to block the sale. "Buyer Beware!", he wrote on his Truth Social platform last month.

OUTCOME OF A BLOCK

U.S. Steel has previously said the deal's failure would put thousands of U.S. union jobs at risk and that it might be forced to close some steel mills. The United Steelworkers union, which opposes the deal, has called those assertions baseless threats and intimidation.

Nippon Steel previously said it was considering all possible measures, including legal action, to close a deal it sees as key to its future growth.