At least 54 Tory MPs have submitted a letter of no confidence to Sir Graham Brady, the chair of the 1922 Committee of backbench Tories - essentially an HR department for backbenchers.The result of the vote will be announced to the public at 9pm - but what scenarios should we be prepared for?
1) The PM wins the vote and continues to lead
The odds are stacked in Mr Johnson's favour as he has a huge majority.
Even though more than 54 MPs called for a vote, it would take 180 of them to agree he should be removed in order to elect a replacement.
If Mr Johnson wins, he will remain as party leader and given a year's immunity from any further confidence votes.
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A major revolt would, however, leave the PM severely damaged. Furthermore, two by-elections on June 23 could demonstrate that Mr Johnson is unpopular with the public, thus delivering further blows.
Would he ever resign off his own back though? ITV News Political Editor Robert Peston says "no Tory MP" thinks Mr Johnson would step down, even if big names in the party urged him to go.
"Nothing in his history suggests he gives a fig what the great and the good think of him," Peston explains.
2) The PM loses and has to leave
If Mr Johnson fails to win over half the votes, he will have to step down as Tory leader and prime minister.
He will be barred from competing in the forthcoming leadership election, but would likely remain in post until a successor is in place.
The leadership contest would take place in two stages. First, Conservative MPs will put themselves forward as candidates and all Tory MPs will vote in a series of rounds to reduce the number of candidates, until only two remain. The second stage of the contest sees the two remaining candidates put to a vote of Conservative Party members.
Mr Johnson losing Monday's no confidence vote may seem unlikely, but as ITV News UK Editor Paul Brand explains, "nothing is certain in this saga".