WASHINGTON (Reuters) -Despite a steady stream of meetings and calls with U.S. officials, and three revamped proposals to assuage national security concerns, Nippon Steel has failed to garner approval from a powerful panel reviewing its $14.9 billion bid for U.S. Steel, a letter seen by Reuters shows.

The letter, sent Saturday, sets the stage for U.S. President Joe Biden, who has long opposed the deal, to block it. The Committee on Foreign Investment in the United States (CFIUS), which reviews deals for national security risks, has a Dec. 23 deadline to approve the deal, extend the review, or recommend Biden scuttle it.

If the agencies that make up the panel remain at loggerheads, as the letter states, they will refer the matter to Biden to take action.

The history of outreach since early September, including four in-person meetings with CFIUS, three phone calls, including one on Friday with the Treasury and Commerce department secretaries, as well as the three proposed mitigation agreements is contained in a letter dated Saturday sent to Nippon Steel by CFIUS that has not been previously reported.

It shows the lengths the companies have gone to try to win approval on the controversial merger, even as the letter signals the deal is likely doomed.

"The Committee has not yet reached consensus on whether the mitigation measures proposed by the Parties would be effective... or whether they would resolve the risk to U.S. national security arising from the Transaction," CFIUS writes in closing.

"The President may take such action for such time as the President considers appropriate to suspend or prohibit a covered transaction that threatens to impair the national security," it adds.

The White House, Nippon Steel, and the Commerce Department, which is co-leading the review of the deal at CFIUS, did not immediately respond to requests for comment. Treasury, which leads CFIUS, declined to comment. U.S. Steel said in a statement that Nippon Steel provides, "by far, the brightest future for U.S. Steel," adding that no other party can make the billions in investments Nippon Steel has promised to make.

"U.S. Steel will not-- and does not have the resources-- to do this on our own," it added.

HIGH-LEVEL OPPOSITION

The proposed tie-up has faced high-level opposition within the U.S. since it was announced a year ago, with both Biden and his incoming successor Donald 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 merger appeared fast-tracked to be blocked after the companies received an Aug. 31 letter from CFIUS, seen by Reuters, arguing the deal could hurt the supply of steel needed for critical transportation, construction and agriculture projects.

But Nippon Steel, countering that its investments, made by a company from an allied nation, would in fact shore up U.S. Steel's output, won a 90-day review extension. That gave CFIUS until after the November election to make a decision, fueling hope among supporters that the calmer political climate could win the deal's approval.

But CFIUS's 29-page letter Saturday, first referenced by the Financial Times and later seen by Reuters, shows the hopes were likely unfounded.