US President Joe Biden has formally blocked the takeover of US Steel by a bigger Japanese company, saying foreign ownership could pose risks for national security.

The controversial decision comes a year after Nippon Steel first announced the $14.9bn (£12bn) deal, describing it as a lifeline for its smaller Pennsylvania-based rival.

But the transaction soon ran into political trouble, after leaders of the United Steelworkers union loudly opposed the deal, bringing political pressure to bear in a key state during the 2024 presidential election.

Biden decided to scrap the deal despite concerns by some advisors that it could damage Washington's relations with Tokyo, a key ally.

BBC News has contacted Nippon Steel and US Steel for comment.

Nippon Steel has previously denied that it planned to reduce production or cut jobs, while US Steel had warned that it might have to close plants without the investment that would come with a new owner.

Those concerns had been echoed by some workers and local politicians.

Other business groups said they feared rejecting the transaction would chill the climate for international investment in the US.

But Biden has voiced longstanding opposition to the deal. The transaction has also been criticised by President-elect Donald Trump and the incoming vice-president, JD Vance.

A US government panel charged with reviewing the deal for national security risks failed to reach a consensus by late December, leaving the decision to Biden, who was required to act within a 15-day deadline.

In his announcement on Friday he said maintaining US ownership was important to keeping the US steel industry and it supply chains strong.

"As I have said many times, steel production - and the steel workers who produce it - are the backbone of our nation," he said.

"That is because steel powers our country: our infrastructure, our auto industry, and our defense industrial base. Without domestic steel production and domestic steel workers, our nation is less strong and less secure."

Nippon Steel and US Steel have previously suggested they may pursue legal action against the government if the deal did not happen.

Prof Stephen Nagy, of the Department of Politics International Studies at the International Christian University in Tokyo, said this was a "political" decision, noting that the Biden administration from its start promised a "foreign policy for the middle class".

"This was a direct response and continuation of the Trump MAGA agenda of Making America Great Again," he said.

"The Biden administration couldn't appear weak on foreign businesses, whether it's an ally or adversary."