His replacement Liz Truss crashed and burned after 45 days in the job, announcing her resignation after being forced to ditch most of her policy programme after it spooked the financial markets.
Mr Johnson won the 2019 general election - and under the British constitution the party in power can change leader without another election.
Ms Truss was elected by Conservative Party members, who may get the final say in this latest contest, if two contenders remain after MPs have voted.
A second Johnson premiership would be an extraordinary turnaround even for a politician who has made miraculous comebacks before.
The last time anyone returned to the office of prime minister after losing the leadership was 140 years ago when William Gladstone returned to lead the Liberal Party.
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