Donald Trump has rejected a joint G7 communique, accusing Canadian Prime Minister Justin Trudeau of being "dishonest and weak".
The US president withdrew his endorsement after departing from the Quebec summit, claiming Mr Trudeau had made "false statements".
Despite tensions at the meeting over Mr Trump's controversial decision to impose tariffs on steel and aluminium from Europe, Mexico and Canada, Mr Trudeau had announced the leaders had managed to agree a statement which highlighted the importance of "free, fair and mutually beneficial trade and investment" and committed the G7 to "continue to fight protectionism".
During the final news conference, Mr Trudeau said Canadians would see Mr Trump's line on trade as "insulting" after a long history of co-operation between the two countries.
He also warned that they "will not be pushed around".
In response, Mr Trump said: "Based on Justin's false statements at his news conference, and the fact that Canada is charging massive Tariffs to our U.S. farmers, workers and companies, I have instructed our U.S. Reps not to endorse the Communique as we look at Tariffs on automobiles flooding the U.S. Market!"
In a second tweet, he added: "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. Our Tariffs are in response to his of 270% on dairy!"
Mr Trump sent the messages from Air Force One just as Canada released the agreed communique, which had been signed by the G7 leaders.
Mr Trudeau's office said in a statement: "We are focused on everything we accomplished here at the #G7 summit."
A senior UK government source added: "We stand by the commitments made in the G7 communique."
As Mr Trump left the summit early on Saturday to catch a flight to Singapore, where he will meet North Korean leader Kim Jong Un on Tuesday he told reporters that his G7 colleagues were starting to be more committed to "fairer trade" with the US.
He said he suggested to them that all trade barriers between their countries - including tariffs and subsidies - be eliminated.
"You go tariff free, you go barrier free, you go subsidy free," he said. "I did suggest it and people I guess were going to go back to the drawing board."
"The United States has been taken advantage of for decades and decades," he said.
Mr Trump said he did not blame the G7 for trade deals but warned "those days are over".
The president said the US was prepared to stop trading with some countries if things did not change - citing India as one country that puts tariffs as high as 100% on American goods.
The summit was dubbed "the G6 plus one" due to the billionaire tycoon's isolation over trade and his decision to pull the US out of the Iran nuclear deal.
Theresa May said there had been some "difficult discussions" and that the debate between leaders had been "open and frank".
Mrs May said they had reached "agreed language" on trade and on Russia but said the EU would still proceed with implementing retaliatory measures to Mr Trump's latest tariffs.
She said the UK had expressed its "deep disappointment at the unjustified decision by the US to apply tariffs to EU steel and aluminium imports" at the meeting.
In a show of unity in their joint statement, the G7 leaders vowed to combat protectionism and cut trade barriers and included commitments to ensure Iran will "never seek, develop or acquire a nuclear weapon".
The statement also included demands for Russia to stop undermining Western democracies - despite Mr Trump repeatedly calling for Vladimir Putin to rejoin the world leaders and form a G8.
There was also an agreement to disagree on climate change following the US president's decision to leave the Paris climate accord last year.
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