The odds of the former chancellor becoming the UK's next prime minister are very short at the moment - but anyone paying attention a couple of years ago could have been seeing a much bigger return on their outlay.
Philip Thompson, a blogger for the Political Betting website, made the case for Sunak - then the Chief Secretary to the Treasury with very little public profile - becoming PM two weeks before the 2019 general election in which Johnson went on to secure a huge majority.
At the time, Sunak had odds of 200/1 at being PM, with Thompson outlining scenarios where Johnson would no longer be leader of the party.
One of those scenarios was “Johnson running into problems early in the next term and has to be replaced by a Tory elder statesmen”.
Among the names tipped to take over were Michael Gove and Jeremy Hunt – with Sunak far behind in the odds.
Labour electing Sir Keir Starmer to replace Jeremy Corbyn was raised as a potential barrier for Johnson winning another election, along with any problems that Brexit might bring about after the 2019 election.
Thompson also raised the possibility of a member of the “next generation” of Tory MPs to replace Johnson instead of the former “big hitters”.
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Outlining his prediction that Sunak would be that person, Thompson wrote how he had been “used frequently on media appearances” during the election campaign, highlighting how he was earmarked by Johnson to represent the Tories on a television debate.
Thompson suggested that Sunak’s 200/1 odds would not remain for long – meaning anyone placing a bet in 2019 will potentially be winning big should Sunak win as currently expected.
What Thompson could not predict was the speed at which Johnson will be replaced – with COVID, Partygate and the Pincher scandal all contributing to the prime minister’s downfall.
Sunak had the backing of 88 MPs after the first ballot on Wednesday evening – more than any other candidate with Penny Mordaunt second with 67.
However, his popularity with Conservative party members is not a sure thing, with a recent YouGov survey finding that rivals Mordaunt and rising star Kemi Badenoch polled higher than Sunak in terms of preference to replace Johnson.
But Sunak leads other rivals, including Jeremy Hunt, Tom Tugendhat and Suella Braverman and he may still secure the premiership – depending on who he faces in the final round.
Despite resigning as chancellor to force Johnson out, Sunak has said he will not “demonise” the PM and insisted the Johnson’s controversial former aide Dominic Cummings has “absolutely nothing” to do with his leadership campaign.
Sunak has also attempted to be realistic about the economy, hitting out at “not credible” tax cut offers made by some of his rivals.
He said that he would only cut taxes “once we’ve gripped inflation”, adding: “We need a return to traditional Conservative economic values and that means honesty and responsibility, not fairytales.”