File photo taken on April 10, 2019 shows then British Prime Minister Theresa May arriving for the special summit of the European Council in Brussels, Belgium. Newly-elected Conservative Party leader Boris Johnson took office as the British prime minister on Wednesday amid the rising uncertainties of Brexit. The latest development came after Theresa May formally stepped down as the leader of the country and Johnson was invited by the Queen to form the government. (Xinhua/Zhang Cheng)