cnn's anna stewart explains. >> reporter: just as boris johnson made a statement of defiance in his debut address as britain's new prime minister -- >> never mind the backstop, the buck stops here. >> reporter: made a statement about his personal life. walking through the famous black door of number 10 downing street alone. an iconic moment usually shared with a prime minister's partner or children. but he wasn't completely alone. boris johnson's girlfriend, carrie simmons, watched from the sidelines. now speculation grows as to whether she will move in, making boris johnson the first prime minister to live unmarried with a partner. >> they are going to make history by being a boyfriend and girlfriend couple moving in to that power base at number 10 downing street. >> reporter: the british media is fascinated by this relationship, given the new prime minister's reputation for a colorful personal life. boris johnson is going through a divorce after a marriage of 25 years and four children. the relationship with simmons,