Trudeau said earlier that all G-7 members had agreed to sign the communique at the Quebec summit.
EU nations are set to impose counter-measures on United States goods in response to tariffs on European steal and aluminium, but Mrs May cautioned against tit-for-tat retaliation that might lead to a trade war.
Trudeau said the Liberal government looks forward to the details of the agreement that emerged from Monday's historic meeting between Trump and North Korean leader Kim Jong Un. "Because Canadians, we're polite, we're reasonable - but we will not be pushed around".
The summit was seen as an opportunity to overcome disagreements on new tariffs imposed by the U.S. on imports.
He also insulted Trudeau personally, dubbing him "very dishonest and weak" and excoriated Canada's managed dairy market, which is a tiny part of total exchanges but is protected by a huge 270 percent tariff.
Canadian Foreign Prime Minister Chrystia Freeland also did not appreciate Trump's comments, saying they were not "appropriate or useful".
He further said that throughout the summit, the United States tried negotiating with Trudeau in "good faith", but the Canadian President undermined the United States and its allies. "Obviously we support the continuing efforts by the President on North Korea", the Canadian prime minister said.
President Trump, meanwhile, was not as kind, accusing Trudeau of "false statements" in a tweet. "I don't understand the obsession with trade relations with Canada", he sighed. Trump reportedly pointed to specific trade deficits, particularly in the trade of goods, as evidence that the other leaders were taking advantage of the US.
Behind his claim lies a difference in how the countries' calculate the trade figures. Nearly 70 percent of the steel used in the United States already is produced here, and our nation can never be self-sufficient in aluminum production because the bauxite used to make the metal is not mined here.
Yet he worked to smooth over the ideological differences and to present himself as the best neighbor the United States could hope for.
Canada is on of the US's largest trade partners, with $673.9 billion worth of goods and services going between the two countries in 2017.
Ms. May ducked Mr. Corbyn's question and when later asked by Conservative MP Crispin Blunt: "Trudeau or Trump?", she smiled and replied: "I'm not sure what activity he's asking me to undertake with either of them".
"According to a Canada release, they make nearly 100 Billion Dollars in Trade with USA (guess they were bragging and got caught!)".
Trump's attacks have Canadian businesses that use aluminum and steel very anxious, said Ontario Conservative MP John Brassard, who added that there is real concern that there will be serious job implications in very short order. "Least of all from subordinates of the president directed at the prime minister of their greatest friend and ally", he said.