The first round of voting among Conservative MPs begins today to determine who succeeds David Cameron as the next prime minister following the Brexit referendum on 23 June.

Theresa May, the Home Secretary, leads the other four contenders: Andrea Leadsom, Michael Gove, Stephen Crabb and Liam Fox.

MPs will vote in a secret ballot today between 11am and 6pm with the results expected around 7pm. The candidate with the least number of votes will be eliminated before MPs vote again on the remaining four on Thursday, and then the last three next Tuesday. After that, the top two candidates will be put to the party's 130,000 members.

May currently has the support of around 115 MPs – around 35 per cent of the parliamentary party – including ten Cabinet ministers. Yesterday, the heavyweight former leadership contender David Davis endorsed May, who also enjoys the support of the Daily Mail. May is currently followed by Leadsom on 40, Gove on 26, Crabb on 23 and Fox on nine.

However, Tory party leadership contests are notoriously unpredictable, with the front-runner sometimes collapsing in later stages of the contest. In 2001, Michael Portillo also started with the support of roughly 100 MPs before being knocked out in the final round of voting ahead of the membership stage. Then, the relatively unknown Iain Duncan Smith beat Kenneth Clarke, easily the most popular Tory among the wider electorate at the time.

Last night, Boris Johnson – who withdrew from the contest after fellow Leave campaigner Gove stood with an attack on Johnson – backed Leadsom, saying she had "the zap, the drive, and the determination" for the job and emphasising her financial experience as a former banker. Today Johnson's centrist brother Jo, also an MP, added his support to May's campaign.

Apart from Crabb, May is the only candidate who did not back leaving the EU. However, there are fears among some moderate MPs that she is subsequently trying too hard to appeal to the Tory right. In an accomplished speech launching her campaign last week, May made a point of emphasising: "Brexit means Brexit." Meanwhile, it is an irony not lost on some MPs that Johnson, who led the Leave campaign, was the candidate who most looked like rowing back on the need for a speedy withdrawal.