Canada, Mexico and the United States will look to dovetail their climate and other policies into a continental strategy when Prime Minister Justin Trudeau hosts Barack Obama and Enrique Pena Nieto on Wednesday.

But Britain's shocking vote to quit the European Union has also abruptly shoved itself onto the agenda of the summit, which the countries tend to hold once a year. Trudeau and Obama had both called on Britain to stay in the EU.

The so-called Brexit decision has shocked the world, roiled financial markets and triggered deep uncertainty over how the divorce will play out, in Europe and elsewhere.

The three countries are economic partners in the North American Free Trade Agreement (NAFTA), and work closely on security matters.

But until recently, both Mexico and Canada have focused on their respective relationship with the United States while largely neglecting each other.

Canada-US ties, meanwhile, soured under the previous Canadian administration over divergent climate goals.

On Wednesday, Trudeau said it was "essential to restore relations with our largest trading partner (the United States) and to fold in Mexico as part of the opportunities for growth and... on climate change."

"We need to make sure that we are engaging in a constructive and productive way with our NAFTA partners," he told a press conference.

Obama and Trudeau have kindled a "bromance" since their first encounter as their country's leaders at an Asia Pacific Economic Cooperation summit in Manila in November, a month after the Canadian won a general election.

It blossomed during a White House dinner in March, when Trudeau agreed to match an existing US target of reducing methane emissions by 40-45 percent below 2012 levels by 2025 from the oil and gas sector.

Trudeau said he would like to see Mexico also match these cuts.

"Climate change and clean growth initiatives are going to be at the heart of many of the discussions will be having... with our continental partners," Trudeau said.

"These and other issues will be things that we are looking to align ourselves, the three partners in NAFTA, as closely as possible," he said.

Six organizations in the three countries said in a report last week the summit provides a rare opportunity to cooperate on climate.

"More than ever before, the United States, Mexico, and Canada are politically aligned on the topic of climate change," said the group, led by the Washington-based Center for American Progress.

"This alignment creates a unique opportunity for the three countries to launch a coordinated climate strategy to propel the shift to clean energy across the continent."

This will be Obama's last North American leaders summit before his presidency ends in January.

The last "Three Amigos" summit was hosted by Pena Nieto in 2014 in Toluca, Mexico.

Obama is scheduled to address a joint session of parliament while in the Canadian capital for the June 29 summit.

"I look forward to the opportunity to speak directly to the Canadian people about the extraordinary future that we can build together," Obama said when the visit was announced in May.

Officials said to also expect investment announcements and a discussion on drug policies, after Trudeau promised to legalize marijuana and the US Drug Enforcement Administration warned of increased overdose deaths linked to Mexican heroin.

Canada is also expected to move to rescind travel restrictions imposed on Mexican travelers nearly a decade ago following a spike in bogus refugee claims.

Mexican Foreign Minister Claudia Ruiz Massieu noted that Trudeau promised in the 2015 election and since forming a government to eliminate the visa requirement.

Trudeau said he looked forward to "fixing some of the challenges that have been going on for too long, like the visa issue."

According to Canadian government figures, Mexican asylum bids have fallen from a peak in 2008 when Mexicans accounted for one in four refugee claims made in Canada, to below one percent.