As Jean-Claude Juncker makes his biggest intervention yet, a look at the history of the EU IBTimes UK

The UK would be regarded as a third-party state by the EU if the British electorate voted to break away from Brussels on 23 June, Jean-Claude Juncker has warned. The European Commission president's intervention is probably his strongest yet and comes with just over a month to go before the historic ballot.

"If the British were to say 'leave', which I am not hoping for, life in the community won't go on as before," he told French newspaper Le Monde. "That is not a threat, but our relations will no longer be as they are today. The UK will have to get used to being regarded as a third-party state."

The comments coincided with an intervention from Justin Trudeau, the latest world leader to back a 'remain' vote. The Canadian prime minister warned there would be "nothing easy or automatic" about negotiating new trade deals after a Brexit.

He also told Reuters: "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.

"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."

Barack Obama issued his own Brexit warning in April when he visited the UK. The US president said Britain would be at the "back of the queue" for any trade deal between the countries as American concentrates on securing an agreement over the Transatlantic Trade and Investment Partnership (TTIP) with the EU.

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. Vote Leave and Grassroots Out had not responded to request for comment in reaction to Juncker's remarks at the time of publication.

Cameron and Juncker
UK Prime Minister David Cameron withJean-Claude Juncker in Brussels Getty