One of two Brexit campaigners is expected to be eliminated in a second round of voting on Thursday by Conservative lawmakers to elect Prime Minister David Cameron's successor.
Home Secretary Theresa May is the clear favourite among the three remaining candidates to become Britain's next leader.
Cameron resigned last month following his failure to persuade voters that Britain should remain in the European Union.
May supported Cameron's Remain campaign but she kept a low profile in the final weeks before the June 23 referendum.
She has vowed to provide "stability and certainty" to the country and unify her divided party.
Her challengers, Michael Gove and Andrea Leadsom, were two of the most prominent Vote Leave campaigners ahead of the referendum.
Thursday's vote by 329 Conservative members of parliament willeliminate the least popular candidate, leaving a shortlist of two candidates for a vote by the party's 150,000 members.
May is expected to be in the final vote against one of her twopro-Brexit rivals.