The ex-PM's arrival came as rival Rishi Sunak reached the minimum threshold set by the conservative party to stand as a candidate.
Boris Johnson has cut short a Caribbean trip to join the race to replace outgoing prime minister Liz Truss, just over a month after he handed over power in early September.
His apparent bid to return to office just weeks later has already been decried by opposition politicians and even some in his own fractured ruling party who argue that both it and the country need stability and unity.
"We've got to go forward, not go back," Johnson's deputy prime minister Dominic Raab told Sky News, adding an imminent parliamentary inquiry into the "Partygate" scandal that dogged his former boss could prove too distracting.
His return came as Conservative leadership rival Rishi Sunak reached the minimum threshold of 100 Conservative MPs to contest the UK's top job.
Raab said former finance minister Sunak's economic experience meant he was the "standout candidate".
However, both Sunak and Johnson are yet to announce they are running, leaving it to allies to signal their intent.
READ MORE: Johnson comeback bid adds drama to UK political crisis
Following Prime Minister Liz Truss’s resignation, the UK’s ruling Conservative Party is expected to elect a new leader next week.— TRT World (@trtworld) October 21, 2022
Here's a look at the likely contenders pic.twitter.com/1ATA7beazc
An accelerated contest
Cabinet member Penny Mordaunt, who just missed out on making the final runoff after Johnson quit, became the first to formally declare her candidacy again on Friday.
The 49-year-old said she was running for "a fresh start, a united party and leadership in the national interest". But she is already trailing her rivals by dozens of nominations.
The accelerated contest will see the Conservatives' 357 MPs hold a vote on Monday on any candidates with the 100 nominations, before a possible online ballot of party members later in the week if two remain.
The Sunak and Johnson camps are reportedly seeking talks to see if there is scope for a unity deal, although there is plenty of bad blood since the former prime minister's ouster.
Sunak's July resignation as chancellor of the exchequer helped trigger the government mutiny that ultimately led to Johnson's ousting.
If found guilty of lying to the Commons over "Partygate", the lockdown-breaching revels held in Downing Street, Johnson could be suspended or even expelled from parliament.
As a result of such controversies, Johnson left Number 10 with dismal poll ratings, and other Tories appear aghast at the prospect of his return.
Veteran backbencher Roger Gale warned that Johnson could face a wave of resignations from MPs refusing to serve under him as leader.
READ MORE: UK's Truss apologises for economic mistakes, will 'stick around'