Justin Trudeau is the latest world leader to intervene in the UK's referendum on the EU and back a "remain" vote. The Canadian prime minister warned there would be "nothing easy or automatic" about negotiating new trade deals if Britain broke away from Brussels after the 23 June ballot.

"There's nothing easy or automatic about that [negotiating a deal], so I think there's a bit of an oversimplification of the story happening there," he told Reuters. The Liberal Party leader also argued that the UK is stronger and more influential on the world stage.

"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," he added.

"I believe we're always better when we work as closely as possible together and separatism, or division, doesn't seem to be a productive path for countries."

Trudeau's intervention is notable since Canada is a member of the Commonwealth of Nations and comes after Barack Obama came out against a Brexit during a visit to the UK in April.

The US president warned that Britain would be at the "back of the queue" for a trade deal with his country, explaining America is hoping to decide on the Transatlantic Trade and Investment Partnership (TTIP) with the EU.

Maybe some point down the line there might be a UK/US trade agreement, but it's not going to happen any time soon because our focus is on negotiating with a big bloc, the EU, to get a trade agreement," Obama said.

Japan's Shinzo Abe, Narendra Modi, the prime minister of India, Chinese President Xi Jinping, and Australia's foreign minister Julie Bishop have also intervened on the side of remain. Brexit campaigners hit back at the interventions by claiming Obama and Abe would not accept losing the same amount of sovereignty the UK has to as a member of the EU.

"Japan wouldn't accept the huge loss of control Britain has suffered because of our EU membership, so much of the public will be sceptical of the Japanese prime minister's 'do as I say, not as we do' attitude," said Matthew Elliott, the chief executive of Vote Leave.

The latest online opinion poll from YouGov, of more than 1,600 people between 16 and 17 May, put remain four points ahead of leave (44% versus 40%), with 12% of respondents undecided.

Justin Trudeau
Canadian Prime Minister Justin Trudeau Nicholas Kamm/Getty