By Kylie MacLellan
ISE-SHIMA, Japan (Reuters) - Former London mayor Boris Johnson could be Britain's next prime minister despite heading the campaign to leave the European Union, Prime Minister David Cameron said.
Last year, Cameron, who has said he will not stand for a third term at a national election due in 2020, cited Johnson, along with interior minister Theresa May and finance minister George Osborne, as potential successors.
Cameron and Johnson now find themselves rivals on opposing sides of an increasingly bitter debate ahead of Britain's June 23 referendum on EU membership.
Many believe Johnson took the gamble that campaigning for "Out" would boost his chances of leading a Conservative Party whose membership is largely euroskeptic.
Asked during a trip to Japan for a G7 summit whether he still believed Johnson could be prime minister, Cameron said he hadn't changed his view.
"The Conservative Party is lucky to have big substantial figures within it and that is certainly the case," he told reporters.
"On this one I think he is on the wrong side ... but I am not changing anything I have said in the past."
A poll earlier this month showed Johnson was doing a better job than Cameron at convincing voters, with only 21 percent of respondents saying the prime minister was more likely to tell the truth about the EU. Forty-five percent said Johnson was more believable than Cameron.
But Johnson has come under fire for some of his comments during the campaign, including making a comparison between the EU and Adolf Hitler's plan to rule the continent.
On Thursday, the EU's chief executive accused Johnson of distorting the truth in trying to persuade Britons to leave the European Union, while a top EU official called possible victories for Donald Trump and Johnson part of a "horror scenario" for the world.
(Editing by Stephen Addison)