UK's new prime minister moves into Number 10

During that final session, she said she was "pleased" to hand over to Johnson, who she said is "a man I worked with when he was in my Cabinet, who is a Conservative and who ran on a manifesto to deliver Brexit".

Speaking outside 10 Downing Street before leaving for the last time, she wished him "good fortune in the years ahead".

"To serve as prime minister of the UK is a great honor," she said. "You achieve nothing alone and so my final words are of sincere thanks to my colleagues."

Johnson is understood to have received several congratulatory telephone calls from world leaders after stepping inside 10 Downing Street.

On Thursday, he will enter the Houses of Parliament for the first time as prime minister, where he will make a statement before fielding questions from MPs. But, he will not have long to get used to the "front benches" before Parliament breaks in the afternoon for a summer vacation that is scheduled to continue until Sept 3.

Cynics are already asking how long his government will last because Johnson inherited a very slim majority and is likely to have alienated some members of his party by helping unseat May.

The opposition Labour Party, the Liberal Democrats, and the Scottish National Party will all oppose him over Brexit and could, with the help of a handful of Conservative Party defections, bring down his government. However, with the smaller parties also seeing a slump in support from voters, they seem reluctant to rush into triggering a general election.

Labour Party MP Barry Gardiner, who is the shadow international trade secretary, said on Radio 4's Today program "discussions are ongoing" between Labour and potential Conservative Party rebels, to see if there is sufficient support to topple Johnson's government, but he said they will not hurry to pull the trigger.