[LONDON] US President Barack Obama is set to enter the Brexit debate when he visits the UK next week, with a clear call for Britain to remain part of the European Union.

Mr Obama believes that while it's a decision for the British people, it's also in the interests of the US, the UK and the rest of the EU to keep the bloc together. A vote to leave would diminish Britain's influence in the world, the president's advisers told reporters on a conference call Thursday.

"If he's asked his view as a friend, he will offer it," Deputy National Security Adviser Ben Rhodes said.

"We believe that all of us benefit when the EU can speak with a strong and single voice, and can work with us to advance our shared interests, whether on security or prosperity."

Mr Obama will arrive in London as part of a trip to Europe and the Middle East just as the Brexit referendum debate shifts up a gear. Friday is the first day of official campaigning ahead of the June 23 vote, with some polls suggesting the outcome is too close to call.

The president will have lunch with Queen Elizabeth at Windsor Castle and hold a joint press conference with Prime Minister David Cameron during his visit.

The UK is "one of the countries that has most shaped the modern era," Senior Director of European Affairs Charles Kupchan said on the call.

"It's our estimate that the United Kingdom will continue to play that role most effectively if it remains part of the European Union."

BLOOMBERG