Cameron's proposals to the European Union include reforming the system of contributions to the bloc's budget, giving national governments more opportunities to block unpopular decisions by Brussels, and providing member states with more powers to control the influx of migrants and distribute social benefits.
"In an EU referendum I would vote to stay in if and only if we get the reforms we need. If you do not have that willingness to walk away you cannot hope to have success in the negotiation," Johnson said, as quoted by the Telegraph.
In March, Cameron said he would only serve two terms as prime minister of the United Kingdom, and named the London mayor among his potential successors. Johnson said it would be an "honor" for him to be considered for the position.
