President Donald Trump has said he will announce a 25% import tax on all steel and aluminium entering the US, a move that will have the biggest impact in Canada.
Trump also said that there would be an announcement later in the week about reciprocal tariffs on all countries that tax imports from the US, but he did not specify which nations would be targeted, or if there would be any exemptions.
"If they charge us, we charge them," Trump said.
The move marks another escalation in Trump's trade policy, which has already sparked retaliation from China.
Canada and Mexico are two of the US's biggest steel and aluminium trading partners.
The US accounts for about 10% of the UK steel industry's exports, but for some suppliers of speciality products it is much more important than that, making up a large proportion of their overall business.
Trade body UK Steel said any US tariffs would be a "devastating blow" to the UK industry.
Steel production in the US is heavily politicised. Some argue the country needs to be capable of producing enough high quality steel to supply its military in the event of a national emergency without relying on imports.
During his first term, Trump put tariffs of 25% on steel imports and 10% on aluminium imports from Canada, Mexico and the European Union.
But the US reached a deal a year later with Canada and Mexico to end those tariffs, although the EU import taxes remained in place until 2021.
Those tariffs increased costs for car firms in the US as well as makers of fizzy drinks who use aluminium in their cans. These were then passed on to consumers, according a report from thinktank Tax Foundation.
Speaking on board Air Force One, Trump said on Monday he would announce tariffs on "everybody" for steel and aluminium.
"Any steel coming into the United States is going to have a 25% tariff," he said.
