May, 59, will succeed David Cameron, who announced he was stepping down after Britons unexpectedly voted last month to quit the EU. Britain’s planned withdrawal has weakened the 28-nation bloc, created huge uncertainty over trade and investment, and shaken financial markets.
May and Leadsom had been due to contest a ballot of grassroots Conservative party members, with the result to be declared by September 9. 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.
Earlier, Cameron told reporters in front of his 10 Downing Street residence that he expected to chair his last cabinet meeting on Tuesday and take questions in parliament on Wednesday before tendering his resignation to Queen Elizabeth.
May will become Britain’s second female prime minister, after Margaret Thatcher.
Her victory means that the complex process of extricating Britain from the EU will be led by someone from the losing side of the acrimonious referendum campaign. She has said Britain needs time to work out its negotiating strategy and should not initiate formal divorce proceedings before the end of the year.
In a speech earlier on Monday in the city of Birmingham, May said there could be no second referendum and no attempt to rejoin the EU by the back door.