In a surprise announcement Johnson, who had been seemingly cast into the political wilderness after the June 23 referendum, was handed a task that will inevitably be dominated by handling Britain's departure from the EU following last month's vote to leave.
Johnson had been widely expected to stand for the leadership of the governing Conservative Party after "Remain" campaign leader David Cameron announced his resignation as prime minister in the wake of the referendum.
Johnson had been the bookmakers' favourite to become prime minister. However, his running mate Michael Gove decided to stand for the job himself and Johnson swiftly pulled out in a shock statement which had been expected to announce his candidacy.