​A billionaire investor and long-time ally of Donald Trump, sold more than $30m of shares in a company that depends heavily on steel, only days before the President announced plans to introduce tariffs on imports of the metal.

Carl Icahn, who served as special economic adviser to the President before he resigned when a magazine article highlighted potential conflicts of interest, got rid of a total of $31.3m of stock in Manitowoc Company Inc. The company describes itself as a leading manufacturer of cranes and requires a large quantity of steel.

Think Progress reported the 82-year-old began selling stocks on February 12, less than two weeks before the Trump administration announced its plan to impose taxes on imported steel and aluminium. It based its report on a filing submitted to the Securities and Exchange Commission on February 22, which showed Mr Icahn sold nearly a million shares of Manitowoc.

Earlier this week, Mr Trump announced he was going to introduce a 25 per cent levy on important steel and 10 per cent on imported aluminium. While some welcomed Mr Trump’s proposal, many both within the US and overseas, criticised them and warned on a potential trade war.

Mr Trump later said a trade war would be a good thing for the US.

“When a country (USA) is losing many billions of dollars on trade with virtually every country it does business with, trade wars are good, and easy to win,” he wrote on Twitter.

“Example, when we are down $100 billion with a certain country and they get cute, don’t trade anymore-we win big. It’s easy.”

The report said Mr Trump’s decision resulted in the shares in steel and aluminium producing firms jumping. Meanwhile, shares of firms heavily depending on the substances, saw major declines. The value of Manitowoc Co fell by more than 6 per cent.

Mr Icahn did not immediately respond to requests for comment.