US President Donald Trump has retracted his endorsement of the joint communique issued at the end of the G7 summit, accusing Canada of "dishonesty".
He said that other countries were imposing "massive tariffs" on the US.
The joint communique, advocating a "rules-based trading system", was reached despite tension over US tariffs on steel and aluminium imports.
Canadian Prime Minister Justin Trudeau vowed afterwards to press ahead with retaliatory tariffs on 1 July.
Speaking at a news conference, he described as "insulting" Mr Trump's decision to invoke national security concerns to justify steel and aluminium tariffs.
"It would be with regret but it would be with absolute clarity and firmness that we move forward with retaliatory measures on 1 July," Mr Trudeau said. "Canadians are polite and reasonable but we will also not be pushed around."
His words contained nothing he had not said before, both in public and in private conversations with Mr Trump, his office said later.
The EU said it would stick to the joint communique despite Mr Trump's decision.
"We stand by the commitments made in the G7 communique," a senior UK government source said.
'Do not endorse'
Tweeting en route to his next summit in Singapore, Mr Trump said he had instructed US officials "not to endorse the communique as we look at tariffs on automobiles".
He said the move was based on Mr Trudeau's "false statements at his news conference, and the fact that Canada is charging massive tariffs to our US farmers, workers and companies".
He suggested Mr Trudeau was "very dishonest and weak".
Mr Trump had earlier signed the joint statement agreed by all the G7 nations despite the trade row.
What is the G7?
It is an annual summit bringing together Canada, the US, the UK, France, Italy, Japan and Germany, which represent more than 60% of global net worth between them.
Economics tops the agenda, although the meetings now always branch off to cover major global issues.
Russia was suspended from the group in 2014 because of its annexation of Crimea from Ukraine. On Friday, Mr Trump made a surprise call for Moscow to be readmitted but German Chancellor Angela Merkel said other members were against the idea.