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.
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.
The G7 summit, held in La Malbaie, Quebec province, also covered such issues as relations with Russia.
In the communique, the group of major industrial nations - Canada, the US, the UK, France, Italy, Japan and Germany - agreed on the need for "free, fair, and mutually beneficial trade" and the importance of fighting protectionism.
"We strive to reduce tariff barriers, non-tariff barriers and subsidies," they said.
What are the tariffs?
On 1 June, the US imposed a 25% tariff for steel and 10% for aluminium on imports from the EU, Canada, and Mexico. Mr Trump said the move would protect domestic producers that were vital to US security.
The EU then announced tariffs on US goods ranging from Harley-Davidson motorcycles to bourbon. Canada and Mexico are also taking action in retaliation.
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.