LONDON -- US President Joe Biden opposing investment by a staunch ally in a US manufacturing company is a sign that protectionism has run amok, London-based daily Financial Times reported on Monday.

In an article titled "Protectionism is running amok in the US," the paper cited a deal reached by Nippon Steel and US Steel in December, when both companies agreed that the Japanese steelmaker acquires its US competitor.

However, the merger is opposed by the United Steelworkers labour union. And on Thursday, Biden sided with the union, declaring that it was "vital" for US Steel to remain "domestically owned and operated."

"Nippon's acquisition of US Steel would benefit the economy broadly and the working class specifically," the article said.

Japan is a staunch ally of the United States. And under the terms of the deal, US Steel production will remain in the United States, it said.

"The president stopped just short of pledging to block the deal, and his statement did not reference national security," it said.

"Foreign investment is a sign of the strength of the American economy, and Biden should be doing all he can to welcome more of it," it said.

Biden's intervention in a single deal is itself a threat to economic liberty, the article said, adding that in a free society, the government should not attempt to blow up voluntary transactions between private parties.