UK's new prime minister moves into Number 10

Meanwhile, the European Commission's newly elected president, Ursula von der Leyen, said she is open to the EU extending the current Oct 31 Brexit deadline.

"There are many different and difficult topics to tackle together," she said. "There are challenging times ahead of us. I think it is very important to build up a strong and a good working relationship because we have the duty to deliver something that is good for people in Europe and in the United Kingdom."

In addition to the UK's relationship with the EU, political observers will be keen to see how the bond between London and Washington changes with Johnson in charge.

Amanda Sloat, a former state department official who now works at the Brookings Institution think tank in Washington, told the Guardian newspaper: "The personal chemistry between the two leaders is going to be much better. (US President Donald) Trump clearly has favored Boris as leader for a while."