File photo taken on May 28, 2019 shows then British Prime Minister Theresa May arriving at the European Union headquarters for an informal dinner of EU heads of state or government 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)