Mr Stewart likened the outgoing Prime Minister to former Italian premier Silvio Berlusconi, Pakistan's Imran Khan and ex-US President Donald Trump, who are all plotting comebacks.
He said Mr Johnson is hoping to "do a Berlusconi" and make a "populist return" to Downing Street after his own MPs ousted him following a string of scandals.
The man who led Britain out of the EU and "got Brexit done" will be replaced by either Liz Truss or Rishi Sunak next week, as the Tory leadership race nears its conclusion.
But Mr Stewart, who stood against Mr Johnson in 2019, says the former journalist hopes to one day return to No 10.
Boris Johnson has been tipped to imitate Italy's Silvio Berlusconi
Wiki Commons/PA
Rory Stewart stood for the leadership of the Conservative Party in 2019
Stefan Rousseau
Mr Stewart said: “He’s trying to do an Imran Khan or a Berlusconi. He’s going to be hovering around, hoping for a populist return.”
Mr Berlusconi, 85, is Italy's former prime minister and has announced his desire to return to politics ahead of elections next month.
Mr Khan, meanwhile is also eyeing a comeback despite losing a no-confidence vote this year.
And while Mr Trump is yet to formally declare his intention to run in the 2024 US presidential election, he is widely expected to do so and remains arguably the most vocal critic of US President Joe Biden's White House.
Mr Stewart fears that Mr Johnson could follow a similar path, and described the outgoing Prime Minister as "dangerous".
He told The Guardian: “I think he is dangerous and there are people out there who want him to come back…
“He’s going to be hovering around, hoping for a populist return.”
Mr Johnson could potentially take another shot at the leadership if his successor, widely expected to be Ms Truss, is brought down by the cost-of-living crisis.
One obstacle could be the Commons Privileges Committee inquiry into whether Mr Johnson lied to MPs over Downing Street lockdown parties.
But although this could further damage his reputation, allies have already denounced the probe as a “witch hunt”.
Mr Stewart, a long-time critic of Mr Johnson, quit the Conservative Party in 2019 after losing the whip for voting to block a no deal Brexit.
He has been appointed president of the GiveDirectly international aid charity.