US tariffs of 50 per cent on steel and aluminium have come into force on Wednesday, a day after President Donald Trump signed an executive order escalating his global trade war.

The new tariffs took effect at midnight on Wednesday, doubling the 25 per cent levies on the sectors that the president had introduced in March.

Duties on UK steel and aluminium will remain at 25 per cent — a carve-out for London after it signed a trade deal with Washington last month.

Trump said on Tuesday that the tariffs were necessary to prevent dumping by foreign producers that would “threaten to impair the national security”.

“The increased tariffs will more effectively counter foreign countries that continue to offload low-priced, excess steel and aluminium in the US market and thereby undercut the competitiveness of the United States steel and aluminium industries,” Trump wrote in the order late on Tuesday.

The additional duties intensify the US president’s bid to reshape international commerce even as much of his trade agenda remains in legal limbo. A court ruled last week that he lacked the authority cited when he imposed his most sweeping “liberation day” tariffs on April 2.

The ruling — which has since been paused by an appeals court — did not affect sector-specific levies, such as those on steel and aluminium, which Trump introduced using a different authority.

Trump indicated he could yet hit London with the higher rate if his commerce secretary Howard Lutnick “determines that the United Kingdom has not complied with relevant aspects” of the agreement after a July 9 deadline.

The president announced his plan to double the tariffs during a rally at a Pennsylvania steel mill last week, promising to erect a “fence” around the domestic metals industry that would in effect lock out foreign producers.