Theresa May will use a meeting of world leaders next week to press US President Donald Trump to reverse tariffs on steel that threaten to spark a damaging international trade war.

The prime minister will travel to Canada for the G7 where she and other European leaders will seek to persuade Mr Trump to back down.

It comes after Ms May branded the tariffs of 25 per cent on steel and 10 per cent on aluminium as “unjustifiable” ahead of the summit, while other leaders vowed retaliatory action.

The row has also cast doubt on the benefits for Britain of seeking a post-Brexit trade deal with the US, with Ms May have failed so far to secure an exemption for UK goods.

Speaking on BBC Radio 4’s Today programme, Liam Fox, the international trade secretary, said the UK was still seeking exemptions based both on specific products and also geographic location.

He said: “We will still be making that case throughout this week. We of course have the G7 leaders’ meeting at the end of the week where the prime minister will be raising this with President Trump alongside other leaders.”

He added: “The United States believes, I believe wrongly and illegally, that this is a national security matter for the US.

Canadian PM Trudeau says new tariffs announced Trump are unacceptable

“Given that we export some pretty complex steel products to the United States which are part of their national security programmes themselves, this reasoning that is given is wrong and therefore we believe unlawful.”

Canadian Prime Minister Justin Trudeau said the tariffs are “totally unacceptable” and a violation of a centuries-old relationship between the US and Canada.

With his country filing two complaints against the US measures and threatening “dollar for dollar” retaliation, he added: “These tariffs will harm industries and workers on both sides of the Canada-US border.”

The EU has said it will impose countermeasures against the US in response to Mr Trump’s tariffs on European steel and aluminium exports.

The bloc’s trade commission Cecilia Malmström said: “Today is a bad day for world trade. We did everything to avoid this outcome.

“Over the last couple of months I have spoken at numerous occasions with the US secretary of commerce. I have argued for the EU and the US to engage in a positive transatlantic trade agenda, and for the EU to be fully, permanently and unconditionally exempted from these tariffs. This is also what EU leaders have asked for.

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“Throughout these talks, the US has sought to use the threat of trade restrictions as leverage to obtain concessions from the EU. This is not the way we do business, and certainly not between longstanding partners, friends and allies. Now that we have clarity, the EU’s response will be proportionate and in accordance with WTO rules.”

The bloc is also taking the US to the World Trade Organisation’s “trade court” – officially known as the dispute resolution mechanism – to get Mr Trump’s policy declared illegal.

Legal proceedings against the US at the WTO started on Friday, with EU member states already having been consulted on Tuesday.