Boris Johnson has said "his job is to serve the people" as he becomes the UK's new prime minister.
Speaking outside Downing Street after accepting The Queen's invitation to form a government, he said he wanted to "change the country for the better".
He said the UK would leave the EU on 31 October "no ifs, no buts", adding: "The buck stops with me."
Mr Johnson took over after Theresa May handed in her resignation to the Queen a short time ago.
The new PM will then begin putting together his cabinet - a number of Mrs May's senior ministers have already resigned, having said they could not serve under him.
As she relinquished power after three years, Mrs May said being prime minister had been "the greatest honour" and wished her successor well.