Gamblers have been piling in to bet on far-right candidate Marine Le Pen wrestling a surprise win over centrist candidate Emmanuel Macron next month, according to bookmakers.
The pair came out top in Sunday's first-round, with Macron taking close to 24% of the vote and Le Pen just over 21%, which will see them face off in May's run-off.
While an average of five polls puts Macron far ahead of Le Pen in the second and final vote, with an average of 61% to her 39%, the odds have been attracting British punters.
Roughly 90% of wagers since Sunday have been placed on a Le Pen win, according to Ladbrokes, while Betfair said 70% of bets have been on the ultra-right candidate, who this week split from the National Front party in an effort to win more votes.
Betfair said £18.3m of bets had been made on the French president market on its betting exchange.
Macron's odds plummeted to a low of 1/8 and have since eased to 1/7, or what the company said was an 88% chance of winning the second round, while Le Pen was on 13/2 to take the premiership.
"Bets being placed on her are likely to be small punts from bettors expecting the type of last minute upset we have become accustomed to in recent politics," said Betfair spokeswoman Naomi Totten.
Ladbrokes had independent candidate Macron at 1/8 but said 5/1 odds for Le Pen had attracted punters.
Spokeswoman Jessica Bridge said: "Bets have been pouring in for Le Pen since the first round of voting on Sunday, however it's Macron who's still attracting the heavy-hitting cash."
French polling experts said Le Pen's chances of becoming president hinged on a low turnout on May 7, with the anti-EU politician hoping to galvanise the 21m people who voted for other candidates or defaced their ballots in the first round, or another 10.6m who abstained.
While political betting has not come close to matching levels wagered on football, the Brexit vote and Donald Trump's election campaign have been good for bookmaker's business.
Betfair saw more than £100m traded on the US presidential election and even more on the Brexit referendum.
After a "disastrous year" for political betting due to Brexit and Trump, rival Paddy Power even recruited Nigel Farage "to revitalise the firm's approach to political betting".