But what are the odds of the former Foreign Secretary and Mayor of London taking the prime position is Theresa May loses the vote? Here's the latest.
Will Boris Johnson be the next Prime Minister?
The bookies currently have Boris at 9/2 to be the next PM, joint favourite with Sajid Javid.
The pair are closely followed by Jeremy Corbyn (at 5/1), should there end up being a general election, and Dominic Raab in fourth (at 11/2).
Now that at least 48 letters have been submitted to the 1922 committee and a vote of no confidence has been called, Johnson's chances of taking the Prime Ministers spot are ever closer.
The Former Foreign Secretary is still a hugely popular figure among Tory grassroots and the general public.
He drew a huge crowd to hear him bash Theresa May's Brexit plans in a barnstorming speech at the Tory conference in Birmingham.
Boris quit as Foreign Secretary on July 9 in a killer blow to May's government.
He walked out just hours after she lost Brexit Secretary David Davis over her Chequers plan to keep close ties to Brussels.
But many MPs appear to have turned against him, meaning he might struggle to get on the ballot.
His aborted leadership bid in 2016 was scuppered by his friend Michael Gove, severely damaging his electoral "brand".
And his hopes could be dashed by reports of a close friendship with a blonde Tory aide and his divorce from wife Marina.
Why did Boris Johnson resign as Foreign Secretary?
Boris quit as Foreign Secretary on July 9 in a bitter blow to Theresa May's government.
He walked out just hours after she lost Brexit Secretary David Davis as she plans to force through a soft Brexit.
The pair, both passionate Brexit supporters, walked out in the wake of the PM's Chequers agreement.
Since leaving Cabinet, the former journalist has regularly spoken out against the PM's Brexit plans and is widely speculated to be considering challenging her for the Tory leadership.
What did Boris say about Theresa May's Brexit plan?
BoJo claimed the PM's Brexit plan would wrap Britain in a "suicide vest" and give the detonator to Brussels.
But Tory rising star and former Army officer Tom Tugendhat, who served in Iraq and Afghanistan, demanded Boris "grow up".
He said: "A suicide bomber murdered many in the courtyard of my office in Helmand. The carnage was disgusting — limbs and flesh hanging from trees and bushes.
"Comparing the PM to that isn't funny."
The former Foreign Secretary quit the Cabinet in protest at the PM's Brexit proposals which she hammered out at Chequers earlier this year.
But he has fallen short of calling for her to resign, despite his position as the favourite to take over.
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