In a string of angry tweets, Trump said the US paid “close to the entire cost of Nato” to help protect countries that “rip us off on trade”.
The US actually contributes about a fifth of Nato’s direct funding under a formula based on national income.
G7 allies criticised Trump’s decision to reject a joint communique, the BBC reported on Monday.
German Chancellor Angela Merkel described his change of heart as “sobering and a little depressing”. The French presidency had earlier condemned “fits of anger”.
After Trump had left the summit in Quebec he rounded on Justin Trudeau personally, suggesting the Canadian Prime Minister was “very dishonest and weak” and “acts hurt when called out”.
During a news conference, Trudeau had reasserted his opposition to US tariffs on steel and aluminium, and vowed to press ahead with retaliatory moves on July 1.
UK Prime Minister Theresa May told MPs on Monday that she wanted to pay “a particular tribute” to Mr Trudeau for his leadership and skilful chairing.
She said it had been “a difficult summit with, at times, some very candid discussions” and that the UK would honour its commitments in the joint communique.
Trump’s latest twitter tirade came after he arrived in Singapore for his summit with North Korea’s Kim Jong-un.
“Fair trade is now to be called fool trade,” he added in response to the threat of the retaliatory tariffs against the US.
The President tweeted on Monday that America was paying disproportionately more towards the costs of the North Atlantic Treaty Organisation (Nato) than fellow members.
“They pay only a fraction of the cost – and laugh,” he wrote, adding that the EU needed to pay “much more” to help fund the international military alliance, which comprises 28 members including the UK, US, Germany, Canada and Turkey. Not all EU members are Nato members.
Trump went on to say that the US gets “unfairly clobbered” on trade despite “protecting Europe at great financial loss”.
“Change is coming!” he warned.
Trump has consistently said that countries that have a trade surplus with the US – meaning they export more to the US than they import – are “taking advantage” of the US.