PARIS (Reuters) - The European Commission has no plan "B" in place if Britain votes to leave the European Union, and the executive body will stay on the sidelines of the referendum campaign, European Union Finance Commissioner Pierre Moscovici said on Sunday.

Britons will vote June 23 on whether to remain a member of the EU. Asked in an interview on France 5 television whether the EU was planning what to do if they vote to leave, Moscovici said, "No, no and no, there is no plan 'B'. It doesn't help us in any way to envisage disaster scenarios."

"The day we start talking about a plan `B' is the day we no longer believe in our plan 'A'. I have just one plan. The United Kingdom in a united Europe," Moscovici said.

Moscovici said the EU's executive will not take part in the referendum campaign, saying any involvement could backfire.

"For me, it is prudent not to go campaign and try to impose a choice on a sovereign people. Referendums are dangerous, especially for Europe," he said.

Asked about the campaign that has kicked off with London Mayor Boris Johnson joining the call for Britain to quit the EU, Moscovici said the move could hurt Johnson's image.

"It will not be easy for Mr. Johnson to end up next to Nigel Farage and some other clowns and populists," Moscovici said.

He said Europe was facing existential challenges, such as the Greek debt crisis and the current refugee crisis, but that the solutions should be at the European level.

"It is a lot of crises, but these are European problems, but they are also international. The idea that you could find national solutions to these problems which are international is a lie," he said.

"If there was a vote for Great Britain to leave the EU, it will be an inversion of the historic dynamic of the past years which has seen more countries join the bloc." Moscovici said.

(Reporting by Bate Felix, editing by Larry King)