Rachel Johnson, the sister of the London mayor, has said that her brother will reveal which camp he's supporting in his Telegraph column at 10pm on Sunday evening. Speaking to Sky News his sister denied that the mayor was "milking it" for publicity, saying it was a "difficult decision for everyone".
David Cameron issued a last-ditch appeal to Boris Johnson not to join the campaign for Britain to leave the European Union.
With the London mayor set to end months of speculation over which side he will back, Mr Cameron said it would be a "wrong step" for Mr Johnson to link up with Ukip leader Nigel Farage and Respect's George Galloway in the "out" camp.
"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," the Prime Minister told BBC1's The Andrew Marr Show.
"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.
"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."