Prime Minister Justin Trudeau says Britons should vote to stay in the European Union in the upcoming referendum that will decide the fate of the United Kingdom's membership in the 28-nation trading bloc.

"More unity is a path toward greater prosperity. We have a great relationship with a strong and united Europe and certainly hope that that continues," Trudeau said in an interview with Reuters. "Britain is always going to have clout, it's just obviously amplified by its strength as part of the EU."

Trudeau has been reluctant to weigh in on the domestic affairs of major allies — taking a largely neutral stand on the ascendancy of Republican presidential candidate Donald Trump, for example — but he took a clear stand against British "separatism" Thursday, which has been dubbed Brexit (or Britain exit).

"I believe that we're always better when we work as closely as possible together, and separatism or division just doesn't seem to be a productive path for countries," he said. "I've spent my life talking about how bringing people together is better, pushing back against divisions."

Ultimately, he said, the decision would be up to British voters to decide their fate.

But he chided former London mayor Boris Johnson for comparing a potentially independent Britain to Canada. The passionate advocate for the "leave" campaign has said Canada is a good example of a country that has free-trade deals with the rest of the world while maintaining control of its borders.

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"I think we can strike a deal as the Canadians have done based on trade and getting rid of tariffs. It's a very, very bright future I see," Johnson has said.

But Trudeau said that was an "oversimplification" of the issue, and that penning trade deals with an independent Britain was not "automatic."

Canada and the European Union have signed a comprehensive trade agreement — something the prime minister called "a great deal for both sides" Thursday — but it has been stalled by legislators at the EU parliament in Brussels. International Trade Minister Chrystia Freeland has made an impassioned plea to European lawmakers in recent weeks, urging them to formally enact the deal.

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Former London Mayor Boris Johnson holds up a Cornish pasty during the launch of the Vote Leave bus campaign, in favour of Britain leaving the European Union, in Truro, UK. (Darren Staples/Reuters)

"You are not going to get a better deal. And this deal will be a very important precedent for progressive trade deals going forward," she said during a recent trip.

The prime minister batted away the suggestion that a vote to leave the EU would bolster Quebec separatism at home. "We haven't seen a spike in interest [in Quebec separatism]. It's not one of those things that an awful lot of people [are talking about]," he said, nothing that many Quebecers are supportive of his government and he is an ardent federalist.

It is not the first time a Canadian politician has waded into a British separation debate. Former prime minister Stephen Harper told a business audience in London in 2014 that it was not in the "greater global interest" for the Scottish people to vote to leave the United Kingdom.

"What would the division of a country like Canada, or the division of a country like the United Kingdom, do to advance solutions to any of these issues?" Harper asked at the time, pointing to issues such as terrorism and climate change.

Scotland ultimately voted against independence.

The Brexit referendum is scheduled to take place on June 23.