It would be hypocritical of President Barack Obama to support Britain staying in the European Union because Washington would never share its own sovereignty, London Mayor Boris Johnson said on Saturday ahead of a visit by the U.S. leader.

Obama is expected to back Prime Minister David Cameron with a show of support for Britain keeping its EU membership on a visit to London next week ahead of an in-out vote on June 23.

Polls show the referendum, with important implications for trade and Britain's status in the world, could be a close call.

"I just find it absolutely bizarre that we are being lectured by the Americans about giving up our sovereignty, when the Americans wouldn't even sign up to the international law of the sea, let alone the International Criminal Court," Johnson, who supports a British exit from the EU, told the BBC.

"They wouldn't dream of sharing sovereignty," he said. "There is an intrinsic hypocrisy ... if that is what he is going to say."

Obama will offer his view as a "friend" that the UK would be better off economically if it stayed in the EU if asked about "Brexit" during his visit, White House officials told reporters on Thursday.

The International Monetary Fund said this week that a vote to leave the EU could deal a damaging blow to the fragile global economy, citing it as a risk alongside slowing Chinese growth.