The person who staked $1.3 million on Biden hasn't been paid out yet

The huge wager was placed October 29 on the Betfair Exchange, the world's largest online betting exchange, where gamblers find others to match their wagers. The US presidential election was by far the biggest single event in the platform's history.

Media outlets including CNN have called the race for Biden, but the bettor can't yet collect their £540,000 ($710,000) profit and original stake. A spokesperson for Betfair said Monday that the "market will be settled once the result has been officially confirmed."

Bets would typically be settled following a result declaration from news networks and a concession speech by the losing presidential candidate, the platform said on Friday. But this year is different. President Donald Trump has not yet conceded defeat and he's continuing to push his attorneys to pursue legal challenges that would delay formal certification of the results.

"We will only settle the markets when there is certainty around which candidate has the most projected Electoral College votes," Betfair said in an article posted to its website on Friday evening. "Before settling the markets, we must therefore wait for clarity around ongoing vote counts, recounts and any potential legal challenges to the results."

The bettor who backed Biden with the third biggest wager in Betfair's history wishes to remain anonymous, the spokesperson for Betfair said Monday.

The US presidential election has shattered records for Betfair.

The contest has so far attracted £567 million ($744 million) in bets, making it the largest event of any kind in the platform's 20-year history. The 2016 US election garnered £199 million ($261 million), for comparison, while £51 million ($70 million) was bet on the 2017 fight between Floyd Mayweather Jr. and Conor McGregor.

— Chris Isidore contributed reporting.