Conservative members of the UK parliament will cast their ballots for one of the 10 candidates between 10 a.m. and noon (09.00-11.00 GMT).
The list of candidates includes ex-foreign secretary Boris Johnson, Secretary of State for Environment, Food and Rural Affairs Michael Gove, ex-leader of the House of Commons Andrea Leadsom, Secretary of State for Foreign and Commonwealth Affairs Jeremy Hunt, former secretary of state for exiting the European Union Dominic Raab, Secretary of State for International Development Rory Stewart, Home Secretary Sajid Javid, Secretary of State for Health and Social Care Matthew Hancock, ex-work and pensions secretary Esther McVee and Member of Parliament for Forest of Dean Mark Harper.
The second ballot, with a threshold of 10 percent, will be held on June 18. On June 19-20, the voting will continue until two top candidates remain.
In the final round, all of the country's Tories will give their final votes via mail ballots and choose the future prime minister.
Outgoing Prime Minister Theresa May resigned on Friday due to the crisis in the Conservative Party and the failure of all her attempts to push her version of the Brexit deal, agreed with the European Union, through parliament. Almost all of 10 candidates for the leader's post are Brexit supporters, with the front-runners backing tough options, including a no-deal one.