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The G7 summit has kicked off with a row after grumpy Donald Trump snubbed Theresa May and launched a furious rant at his allies.

The US President blasted leaders of the world's richest nations on Twitter as they prepare to confront him over his 25% tariffs on steel.

The UK, US, France, Italy, Germany, Japan and Canada are set for a frosty meeting after Trump slapped the tariffs on imports from the EU, Canada and Mexico.

British Prime Minister Mrs May insists the tariffs, along with 10% fees on aluminium, are "unjustified" amid fears of a trade war.

Yet overnight it emerged Trump is the only G7 leader Mrs May won't meet formally at the two-day summit in Quebec.

(Image: REX/Shutterstock) (Image: REX/Shutterstock)

She wants informal talks with the US President, but he's expected to make a shorter trip to the summit than his fellow leaders, leaving little time for diplomacy.

And pair's relationship is already strained after one former Trump aide told the Telegraph: "She is basically a school mistress.

"I’m not sure anyone gets on well with her."

Trump set the tone overnight by lashing out at "so indignant" Canadian PM Justin Trudeau and saying measures backed by French President Macron were "killing our Agriculture!"

The US President wrote on Twitter: "Please tell Prime Minister Trudeau and President Macron that they are charging the U.S. massive tariffs and create non-monetary barriers.

(Image: PA) (Image: REX/Shutterstock)

"The EU trade surplus with the U.S. is $151 Billion, and Canada keeps our farmers and others out. Look forward to seeing them tomorrow.

"Prime Minister Trudeau is being so indignant, bringing up the relationship that the U.S. and Canada had over the many years and all sorts of other things.

"But he doesn’t bring up the fact that they charge us up to 300% on dairy - hurting our Farmers, killing our Agriculture!"

Macron stoked the flames by making clear he would leave the US isolated if needed.

“No leader is forever,” he said.

(Image: REX/Shutterstock)

"Maybe the American president doesn't care about being isolated today, but we don't mind being six, if needs be."

Speaking to reporters on her flight last night, UK Prime Minister Theresa May said she wanted a "proportionate" response from the EU.

She added: "I made my views clear on the steel and aluminium tariffs that President Trump has announced. I have done that directly to him."

Canadian Prime Minister Justin Trudeau predicted "robust discussions" on trade.

But other G7 members like Japan and Italy seemed less likely to want to challenge the US president amid fears it could worsen a trade war.