Prime Minister David Cameron on Sunday implored Boris Johnson not to join the campaign for a British exit from the European Union as the charismatic London mayor prepares to declare his stance on membership.

Cameron called a June 23 referendum after clinching a deal from other EU leaders that he said gave Britain special status in the world's biggest trading bloc, though six of his cabinet have rebelled and are campaigning to leave the EU.

Johnson, who has charmed some voters with a buffoonish persona that masks fierce ambition to succeed Cameron, has so far been silent though British media are speculating that he 51-year-old mayor will join the campaign to leave the EU.

Cameron cautioned Johnson against joining with opponents of the EU such as UK Independence Party chief Nigel Farage and maverick campaigner George Galloway.

"The prospect of linking arms with Nigel Farage and George Galloway and taking a leap into the dark is the wrong step for our country and if Boris, and if others, really care about being able to get things done in our world then the EU is one of the ways in which we get them done," Cameron said.

"I would say to Boris what I say to everybody else, which is that we will be safer, we will be stronger, we will be better off inside the EU," Cameron told the BBC.

The mayor is due to publish his views on Britain's EU membership at 2200 GMT (1700 ET) on Sunday.

An Ipsos MORI poll showed Johnson, 51, is second only to Cameron when it comes to swaying public opinion on Europe. One in three voters said Johnson would be important in helping them decide which way to vote, the poll showed.

Johnson has asked Cameron to give additional guarantees that the British Parliament is sovereign over EU laws, though it is unclear how Cameron will do that.

"We are going to set out in the coming days proposals... to make clear that British parliament is sovereign," Cameron told the BBC but refused to give further details.