Supporters of the hopefuls are counting people in and out of the room in Parliament where voting is taking place. As political correspondents outside note, for some it looks like it'll come down to the wire.

The frontrunner in the contest, Boris Johnson, has also cast his vote - but did not respond to any questions from the journalists waiting outside.

Remember, the magic number is 33.

Sajid Javid has cast his vote, shortly followed by Dominic Raab, who received 27 votes in the last week's ballot so needs at least another six to meet the minimum threshold to stay in the contest.

He told reporters yesterday that he was "quietly confident" of getting to that point today.

That must be the buzz phrase of the moment... Mr Javid, too, is "quietly confident" according to the Sun's political correspondent.

Our producer in Parliament informs us that Rory Stewart, currently sixth-placed in the contest, has arrived to cast his ballot.

He only got 19 votes in the first ballot, so he needs to win quite a few colleagues over if he is to make it through to the next round.

Also arriving to vote are former contenders Mark Harper and Andrea Leadsom, who were both knocked out after last week's ballot.

Laura Kuenssberg
BBC political editor

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As voting gets under way, our producer on the ground says a small team of Boris Johnson supporters has arrived outside the room where it is being held, taking a tally of MPs as they go through.

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Voting is due to get under way at 15.00 BST in Committee Room 14 of the House of Commons.

It's a secret ballot, meaning MPs can in theory vote differently to how they have declared publicly they will do so.

The mechanics of the event are fairly traditional - there's no electronic voting, with MPs entering to put their ballot papers into a black metal box.

It's all being run by the officers of the 1922 Committee, made up of backbench Conservative MPs.

Dame Cheryl Gillan and Charles Walker are running the show as co-chairs and returning officers, after 1922 chair Sir Graham Brady recused himself from the process.

Read more here.

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Rory Stewart has been the surprise package of the contest. It's not clear whether he's got the numbers to get through to the next round - this morning he said things were "on a knife edge" for him. But as one political commentator - Steve Richards - and one Stewart backer - author Robert Harris - say, he's viewed by some as offering a different potential future for the Conservatives.

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There are six contenders left in the race to acquire the keys to No 10 - after three MPs were eliminated from the contest last week, and Health Secretary Matt Hancock dropped out:

Boris Johnson - former Foreign Secretary and Mayor of London

Jeremy Hunt - current Foreign Secretary and former Health Secretary

Michael Gove - current Environment Secretary and former Education Secretary

Dominic Raab - former Brexit Secretary and former Justice Minister

Sajid Javid - current Home Secretary and former Housing Secretary

Rory Stewart - current International Development Secretary and former Prisons Minister

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Robert Halfon, who is backing Home Secretary Sajid Javid for the leadership, says he is "confident Sajid is going to do well this afternoon".

He says a "significant number" of MPs have moved over to their camp since the first ballot.

Mr Javid, he says, "reaches out to all kinds of different communities" - and has made the point the eventual head-to-head cannot look like the "semi-final of University Challenge".

There needs to be a "real contest that reflects the best of the Conservative Party", he adds, and the home secretary "embodies modern Britain".

A bit more on the debate format. The programme at 20:00 BST on BBC One hosted by Newsnight presenter Emily Maitlis.

Called Our Next Prime Minister, it will see the candidates face questions from viewers via video links from BBC studios across the UK.

The BBC received thousands of questions ahead of the debate, with the largest number being about Brexit.

Other topics submitted included HS2, public spending and climate.

Each candidate will be given an opportunity to answer every question posed.

The programme team will select the questions to be asked after making an editorial decision on the ones that best represent the issues likely to face the next prime minister.

Read more.

Our correspondent Nick Eardley visited the debate studio in central London earlier. His tweeted this picture as the set was being built and the chairs were still in their wrapping.

There are five seats at the moment, but as we said, it's possible more than one candidate could be eliminated in this afternoon's ballot.