“Based on Justin’s false statements at his news conference, and the fact that Canada is charging massive Tariffs to our US farmers, workers and companies, I have instructed our US Reps not to endorse the Communique as we look at Tariffs on automobiles flooding the US Market!” Trump tweeted.

“PM Justin Trudeau of Canada acted so meek and mild during our @G7 meetings only to give a news conference after I left saying that, ‘US Tariffs were kind of insulting’ and he ‘will not be pushed around.’ Very dishonest & weak.”

Earlier, Trudeau had told reporters that Trump’s decision to invoke national security to justify US tariffs on imports of steel and aluminium was “insulting” to Canadian veterans who had stood by their US allies in conflict’s dating back to the first world war

“Canadians are polite and reasonable but we will also not be pushed around,” he said.

And he said he had told Trump “it would be with regret but it would be with absolute clarity and firmness that we move forward with retaliatory measures on July 1, applying equivalent tariffs to the ones that the Americans have unjustly applied to us.”

After Trump’s angry tweets, Trudeau’s office issued a brief response: “We are focused on everything we accomplished here at the G7 summit. The Prime Minister said nothing he hasn’t said before – both in public, and in private conversations with the President.”

Despite the tension and determination of European leaders President Emmanuel Macron of France and Chancellor Angela Merkel of Germany to push back on Trump’s assault on the world trade system, when Trump left it was thought a compromise had been reached.

Officials from European delegations quickly leaked copies of the joint statement, and it was published publicly online before Trump tweeted.

Copies that begin “We, the Leaders of the G7” were distributed in the press room stamped “Approved”.

But Trump’s outburst suggested that any deal had collapsed, and his more or less explicit threat to impose sanctions on imports of cars will outrage his ostensible allies – in particular Germany and Canada who produce many for the large US market.

In retrospect, the consensus had appeared shaky from the outset, and even as Trump flew out it was clear that the summit had failed to heal the rift on trade.

Trump claimed America had been obliged to levy the metals tariffs as it has been exploited as the world’s “piggy bank” under existing arrangements, but his counterparts were equally determined to protect “rules-based” international trade.

The joint communique that was thrashed out over two days of negotiations vowed that members would reform multilateral oversight of commerce through the World Trade Organisation (WTO) and seek to cut tariffs.

“We commit to modernise the WTO to make it more fair as soon as possible. We strive to reduce tariff barriers, non-tariff barriers and subsidies,” it said, reflecting the typical language of decades of G7 statements.

Trump had already said he would not hesitate to shut countries out of the US market if they retaliate to his tariffs.

“The European Union is brutal to the United States ... They know it,” he insisted in his departing news conference. “When I’m telling them, they’re smiling at me. You know, it’s like the gig is up.”

European officials said Trump had tried to water down the language in the draft communique on the WTO and rules-based trade. In the end, that language stayed in and it was only on climate change that no consensus was reached.

European leaders had put a brave face on the results.

“For us, it was important to have a commitment to rules-based trade,” Germany’s Chancellor Angela Merkel said.

French President Emmanuel Macron, whose country will host next year’s summit in Biarritz, said work would continue.

At the summit, Trump played a wild card, suggesting that rather than both sides boosting retaliatory tariffs – as he has just done on steel and aluminium – they could declare for entirely free trade in the G7 zone.

“That would be the ultimate thing, whether or not that works, but I did suggest it,” he said, triggering scepticism from European officials.

The summit was wrapping up just as Chinese President Xi Jinping began hosting the leaders of Russia and Iran at a regional security meeting in a symbol of the power-play between East and West.