David Cameron has warned Boris Johnson that if he "cares about getting things done in our world" then he should opt for Britain to stay in the European Union .
The Mayor of London, as well as Uxbridge and South Ruislip MP, will finally announce at 10pm on Sunday (February 21) which side he is joining in the EU fight.
Reports claim undecided Mr Johnson will back the Leave campaign.
Speaking on BBC 's Andrew Marr Show, the Prime Minister 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.
"I think 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."
Boris Johnson's decision could split the Conservative party even more after six ministers - Michael Gove, Iain Duncan Smith, Chris Grayling, Theresa Villiers, John Whittingdale and Priti Patel all joined the Leave campaign, as well as Mayoral candidate Zac Goldsmith.
