The backlash against Donald Trump’s tariffs grew on Wednesday as steel users challenged them in court and carmakers warned of a surge in prices if the US president delivered on threats to extend them to more valuable auto imports. 

In the most significant legal challenge so far to Mr Trump’s trade policies, the American Institute for International Steel said it had lodged an action in the US Court of International Trade to have the 1962 statute that Mr Trump used to impose steel tariffs ruled unconstitutional. 

The president last year invoked a provision known as “Section 232” of the law to launch an investigation into whether steel and aluminium imports posed a threat to US national security. Last month he  launched a similar probe into imports of cars and parts from around the world worth more than $330bn last year.