Tory MPs are voting in the first round of the contest to select a new party leader and the next prime minister.
A secret ballot is being held in the House of Commons, with a result expected some time after 13:00 BST.
Outgoing leader Theresa May refused to say which of the 10 contenders she had voted for, telling reporters: “That’s none of your business.”
Any candidate who fails to secure at least 17 MPs’ votes will be eliminated from the contest.