The Conservative party's 313 MPs are voting in the secret ballot to replace outgoing leader Theresa May, in a contest that has been marked by the past drug confessions of leading candidates
At least one of the six candidates vying to become Britain's next prime minister will be eliminated in the second round of voting in parliament on Tuesday, with the final pair to be decided by the end of the week.
The outcome of the Conservative party leadership battle could determine under what conditions Britain leaves the European Union. Brexit is currently scheduled for October 31.
All eyes are on whether frontrunner Boris Johnson can maintain his strong lead from the first round of voting, and whether dark horse Rory Stewart can cause a shock by taking out one of the other big hitters.
The Conservative party's 313 MPs are voting in the secret ballot to replace outgoing leader Theresa May, in a contest that has been marked by the past drug confessions of leading candidates.
Further rounds scheduled for this week will whittle the list down to just two, who will face the party's 160,000 grassroots members.
The winner of the race, which also includes Foreign Secretary Jeremy Hunt and Environment Secretary Michael Gove, will automatically become prime minister.
Johnson, the former foreign minister and leading Brexit campaigner, won the support of 114 MPs in the first round of voting last week - more than twice the number garnered by his nearest challengers.
He has said Britain must leave the EU on October 31, ideally with a formal deal between London and Brussels, but if necessary without one.
Opponents of "no-deal" say it would cause economic chaos as Britain breaks up with its top trading partner.
Last two before Friday
Candidates must secure the support of at least 33 MPs in Tuesday's ballot. If they all reach the threshold, the worst performing contender will drop out.
The result will be announced around 1700 GMT.
Two hours later, the remaining candidates will do battle in a BBC televised debate, following a first TV showdown on Sunday that Johnson skipped.
More rounds of voting are scheduled for Wednesday and Thursday, before the party membership vote on the final choice. The winner will be announced in the week starting July 22.
Hunt finished second last week with 43 votes, followed by Gove on 37, Brexit hardliner Dominic Raab on 27, interior minister Sajid Javid on 23 and International Development Secretary Stewart 19.
The unconventional Stewart has earnt praise for his performance in Sunday's Channel 4 debate and for his strong campaign on social media, reaching out to centre-ground voters from different parties.
Stewart told BBC Radio on Tuesday that he had "no problem" with being seen as the "stop Boris" candidate, accusing the frontrunner of making Brexit promises he couldn't keep.
'Get us out of the EU'
Four candidates have said they will seek to renegotiate the draft Brexit divorce deal agreed with Brussels - even though EU leaders have repeatedly ruled this out.
Only Stewart said he would press ahead with the current agreement, which has been rejected by parliament three times.
The four candidates without Stewart said they would be willing to lead Britain out of the European Union without a divorce deal on October 31 - the current deadline set by the EU.
Johnson has also said Brexit must happen on that date, "deal or no deal".
He received a boost before Tuesday's vote when Andrea Leadsom, the former leader of the House of Commons who was eliminated in the first round of votes, pledged her support.
"He is the best placed to get us out of the EU in October," she said.
Despite finishing sixth in the first round, Stewart has picked up backing from Remain-supporting MPs, including May's deputy prime minister David Lidington, leaving Javid in a precarious position.
The interior minister took aim at rival Stewart on Tuesday, saying "he is effectively telling us we should remain in the EU."
Johnson has come under fire from his rivals for giving few interviews and public appearances, but will appear in Tuesday's TV debate.