WASHINGTON (Reuters) -Nippon Steel's ( NISTF ) $15 billion bid for U.S. Steel has been referred to U.S. President Joe Biden, a White House spokesman said, giving the president 15 days to decide on a tie up he has previously said he opposes.

The Committee on Foreign Investment in the United States (CFIUS), which reviews foreign investments in the U.S. for national security risks, referred the deal to Biden after it was unable to reach a consensus, the companies and two sources said.

That gives Biden, who has long opposed the tie-up, 15 days to block the deal. If he takes no action in that time, the merger would get an unexpected greenlight.

"We received the CFIUS evaluation and the President will review it," White House spokeswoman Saloni Sharma said.

The referral to Biden paves the way for the deal to be killed before President-elect Donald Trump takes office.

Trump, who will be inaugurated on Jan.20, has also opposed the deal, which was first announced last December.

Nippon Steel ( NISTF ) and U.S. Steel said on Tuesday they were informed of the referral. Both companies have previously said they had planned to close the deal before the end of 2024.

"We urge him (Biden) to reflect on the great lengths that we have gone to address any national security concerns that have been raised and the significant commitments we have made to grow U. S. Steel," Nippon Steel ( NISTF ) said in a statement.

U.S. Steel said: "It is our hope that President Biden will do the right thing and adhere to the law by approving a transaction that so clearly enhances U.S. national and economic security."

If the deal collapses, Nippon Steel ( NISTF ) must pay a $565 million penalty to U.S. Steel. It has earlier said it could pursue legal action against the U.S. government if the deal falls apart.

Nippon Steel ( NISTF ) aims to raise its global steel production capacity with U.S. Steel to 85 million metric tons per year from 65 million tons and the deal is core to its goal of lifting production to more than 100 million tons in the long-term.

The U.S. is the only developed nation where domestic steel demand is increasing, with the highest steel prices globally due to production capacity falling short of domestic needs, SBI Securities analyst Ryunosuke Shibata said.

That makes U.S. Steel "a once-in-a-lifetime opportunity for Nippon Steel ( NISTF )," Shibata added.

LACK OF CONSENSUS

CFIUS said on Monday that allowing Nippon Steel ( NISTF ) to take over U.S. Steel could result in lower domestic steel production representing "a national security risk", according to the Washington Post, which first reported the referral to Biden.

Nippon Steel ( NISTF ) said it could eliminate that risk by appointing U.S. citizens to top management and board of director positions at U.S. Steel, but the committee was divided in its view of whether those remedies would be sufficient, said the newspaper.

The U.S. Treasury Department, which leads CFIUS, and the Commerce Department, declined to comment.

The proposed tie-up has faced high-level opposition within the U.S. since it was announced, with both Biden and Trump taking aim at it as they sought to woo union voters in the swing state of Pennsylvania, where U.S. Steel is headquartered. The president of the United Steelworkers Union opposes the tie-up.

"The question is, what will Biden's decision be? And I think that's still very unpredictable," said Nick Wall, M&A partner at Allen & Overy. "He's got nothing really to lose."

In August a CFIUS letter sent to the companies, seen by Reuters, which said the deal could hurt the supply of steel for critical transportation, construction and agriculture projects.

But Nippon Steel ( NISTF ) won a 90-day review by countering that its investments made by a company from an allied nation would in fact shore up U.S. Steel's output.

That gave CFIUS until after the November U.S. election to make a decision, fueling hope among supporters that a calmer political climate could underpin the deal's approval.

Those hopes were dashed last weekend when CFIUS sent a 29-page letter to the companies raising allegedly unresolved national security risks, Reuters exclusively reported.

The companies countered in a follow-up letter, exclusively reported by Reuters Friday, that Biden had "impermissible influence" over the national security review process, threatening legal action if the deal is blocked.