David Cameron has gone out not with a bang, nor a whimper, but a mystery tune.
"We now need a new prime minister in place as soon as possible", Leadsom said. One writer opined that Cameron had just unwittingly revealed the secret "Open Sesame" musical code for the Downing Street door. Some thought they detected a hint of "The West Wing" theme tune.
Labour election co-ordinator Jon Trickett has said he is putting the whole party on a general election footing, on the same day Angela Eagle launched a bid to get rid of leader Jeremy Corbyn.
May addressed her supporters amid a dramatic and turbulent day, trying to reassure the public with characteristic grit and poise.
May backed the "Remain" camp during the campaign for Britain's referendum on European Union membership on June 23 but has made clear since then that it must now go ahead, saying: "Brexit means Brexit".
And she set out her One Nation vision of "a country that works not for the privileged few, but that works for every one of us", adding: "We are going to give people more control over their lives and that's how together we will build a better Britain".
The 59-year-old became the leader in waiting when her only rival withdrew from the race, citing a need for national stability and continuity following the vote.
In announcing Monday that she was abandoning her pursuit of the Conservative Party leadership, Leadsom said that she would support May.
He said: "She is more than able to provide the leadership that our country is going to need in the years ahead and she will have my full support".
May, who has been Home Secretary, or interior minister, since 2010, has won plaudits from other party members while in the job.
She has also been described by a Conservative Party grandee, Ken Clarke, as a "bloody hard woman".
Conservative leadership contender Andrea Leadsom, issues a statement outside her home in Northamptonshire, England, after a newspaper suggested she was using her status as a mother to gain an advantage over leadership rival Theresa May, Saturday July 9, 2016.
British Prime Minister David Cameron is to resign Wednesday, paving the way for Home Secretary Theresa May to take the reins.
The queen has the formal role of appointing the new leader.
"Brexit" refers to Britain's decision to withdraw from the European Union, decided in a referendum on June 23. "I have concluded that the interests of our country are best served by the implementation of a strong leader", Leadsom said.
"The aim is to create here in Britain a really hostile environment for illegal migration", May said in 2013. She faces the possibly insurmountable challenge of negotiating a new relationship with the European Union that can satisfy Brexit-supporting Britons, disappointed "remain" voters and the 27 remaining members of the bloc.
The head of the committee running the Conservative leadership contest says it and the party board will meet to discuss confirming Theresa May as the victor of the race and Britain's next prime minister.
"She was completely Stakhanovite in work. she was very organised but also she had clear priorities".
Mrs May was later confirmed as the new Tory leader by the chairman of the party's 1922 backbench Committee, Graham Brady.
He did not suggest re-opening the contest, saying May is now the only contender.
May's accession is unlikely to end the political turbulence. The referendum result sent the pound to a 31-year-low and markets pummeling.
She said Britain needed a government that would "move quickly to set out what an independent United Kingdom's framework for business looks like".
The new prime minister faces a long painstaking to-do list. She says the country needs certainty, not a nine-week leadership race.
The party is expected to say whether it will to re-open the contest to candidates eliminated in earlier rounds, or declare May the victor unopposed. If so, May could be prime minister within days. But Leadsom unexpectedly quit on Monday after a campaign dogged by ill-judged comments about her rival's lack of children and questions about whether she had exaggerated her CV.
Cameron announced his resignation last month because he backed the losing side in a referendum for Britain to leave the European Union.
Leadsom told The Daily Telegraph newspaper that she believed that having children has "no bearing on the ability to be PM". May said Monday that she had accepted the apology.
The energy minister has drawn fierce criticism in the past week, accused of exaggerating her professional experience and asserting she could run the country better than May because she is a mother.