1 July 2016 Last updated at 11:22 BST
The Leave campaigner, who shocked Westminster on Thursday by pulling out of the Tory leadership race, said: "It's going to be up to someone else now."
Asked what he thought of his fellow Leave campaigner Michael Gove's decision to run instead, Mr Johnson added: "I wish him every possible success."
Boris Johnson has been quizzed by reporters as he left his home in London this morning.