LONDON (Reuters) - The price of copper in the United States rocketed on Monday after U.S. President Donald Trump pushed forward his plans to slap tariffs on industrial metals.

The premium of U.S. Comex copper futures over those traded on the London Metal Exchange surged to a record of $920 per metric ton by 1820 GMT, up from $558 on Friday.

Trump was expected later on Monday to slap new 25% tariffs on all steel and aluminium imports, on top of existing metals duties, in another major escalation of his shake-up of trade policy.

Last week Trump said he also planned to impose tariffs on copper, without giving details.

Since Trump became president, Comex copper has traded at a premium to the LME, ranging from about around $250 to $500 as investors sought to price in the impact of potential tariffs.

But the gap surged on Monday as traders scrambled to catch up with the news flow.

"The price arbitrage between the COMEX and the LME ballooned to an all-time high today as traders continued to price in the implementation of copper import tariffs to the US," Benchmark Minerals Intelligence said in a note.

The premium implied that the market was pricing in the equivalent of a 10.5% tariff on copper, it said.

The previous peak in the premium was in late May 2024, when it hit $655 a ton as Comex copper hit an all-time high of $5.1985 a lb due to a short squeeze.

At the time, parties were forced to buy back their short positions at a loss or deliver physical copper to close them out.