Canada allowed the US National Security Agency (NSA) to spy on G20 talks in Toronto in 2010 and at the G8 summit days earlier, documents leaked by whistle-blower Edward Snowden reveal.
The papers detail a six-day spying operation run out of the US embassy in Ottawa, said the Canadian Broadcasting Corporation (CBC).
The monitoring - conducted while Barack Obama and 25 other foreign heads of government met on Canadian soil in June 2010 - was meant to "support US policy goals".
The G20 summit in Toronto focused on how to prevent another financial crisis.
One proposal included a global tax on banks, an idea strongly opposed by both Washington and Ottawa, and which was eventually scratched.
G8 leaders met in Huntsville, 220 kilometres north of Toronto, before the G20 meeting.
The documents cited by the CBC said the US spying operation was "closely co-ordinated with the Canadian partner", the Communications Security Establishment Canada (CSEC).
By law, the CSEC cannot target anyone in Canada without a warrant and is prohibited by international agreement from getting the NSA or others to spy on its behalf.
The CSEC said the agency "does not target Canadians anywhere or any person in Canada", nor would it ask international partners to "act in a way that circumvents Canadian laws".
The official noted that CSEC activities are reviewed by an independent commissioner who for the past 16 years has reported that the agency "continues to act lawfully in the conduct of its activities".
The CSEC and the government declined to comment on the operations or capabilities of Canada's allies.
The statements were echoed by the CSEC chief John Forster and Canada's defence minister, Rob Nicholson, but that did not satisfy the opposition. New Democratic Party leader Tom Mulcair said in Parliament: "We know it is prohibited. The question is, 'Did they do this, yes or no?' Did (the CSEC) help the Americans to spy on us in Canada during the G20 summit?"
US officials in Washington were not immediately available for comment.
Australia, Britain, Canada, the United States and New Zealand are party to a decades-old signals intelligence sharing agreement known as the Five Eyes.
Documents previously released by Snowden - the former NSA contractor who is now a fugitive in Russia - exposed its spying on world leaders, causing diplomatic disputes between the US and its Western allies.
Data protection has suddenly become a hugely sensitive topic, as highlighted by the reported US tapping of German Chancellor Angela Merkel's mobile phone.
An uproar over the mobile phone tapping prompted the European Parliament to call for talks on a massive free-trade deal with the US to be scrapped. The talks resumed earlier this month after US Secretary of State John Kerry said that in some cases, US spying had gone too far.
"I assure you, innocent people are not being abused in this process, but there's an effort to try to gather information," Kerry said. "In some cases, it has reached too far inappropriately."
On Tuesday, a UN rights committee passed a "right to privacy" resolution pressed by Germany and Brazil, which have led international outrage over reports of US spying on their leaders.
French Prime Minister Jean-Marc Ayrault has said he was "deeply shocked" by claims that the US intercepted millions of phone calls in France.
Mexico's President Enrique Pena Nieto also lashed out at claims the NSA delved into his personal e-mail account.
The Guardian newspaper has said Britain snooped on delegates of the Group of 20 during two gatherings in London in 2009, hoping to get an unfair advantage in negotiations and policy debates.