One of the world's biggest bookies is so sure that Hillary Clinton will win next month's U.S. presidential contest that it has already started paying bettors.

Paddy Power said Tuesday it will begin to pay out more than $1 million US to people with bets on its books in favour of Clinton, whose odds of winning have risen so high that a bettor would have to put up $11 just to win $2. In decimal terms, that implies the wife of former president Bill Clinton has better than an 84 per cent chance of victory.

That's in keeping with some recent polls, with Nate Silver's FiveThirtyEight.com currently predicting an 85 per cent chance of victory for the Democrats, based on a compendium of different polls.

"With national polls showing a healthy lead for the Democratic candidate and Donald Trump's campaign running into scandal after scandal, Paddy Power believes it's a done deal and that Hillary is a nailed-on certainty to occupy the Oval Office," Paddy Power, based in Ireland, said about the Republican nominee.

In pure financial terms, the company has little incentive to pay out early, as the wagers will cost the same no matter when they are paid. But Paddy Power is gambling on drumming up some new business by publicizing the $1 million the company bets it was going to have to pay out anyway in the event of a Clinton victory.

From longshot to legitimate

Trump first became an option on the site's presidential betting book in 2012, where he started as a 100-1 longshot. He stayed at those odds until June of 2015, shortly before he formally launched his candidacy.

Eleven months later, his odds had improved to as high as 13-8, "but recent revelations have halted his momentum and caused his chances of victory to plummet like the value of sterling, resulting in chunky odds of 9-2 — representing a 18.2 per cent chance of winning," Paddy Power said.

The website is still accepting bets on the outcome of the Nov. 8 vote, but any bets already on the books are eligible to be paid out.

The website took a similar tack in 2012, opting to pay out $700,000 to bettors who had put their money on Barack Obama. That time, however, the site didn't make the offer until two days before the vote. In the current campaign, there's still 21 days to go.

The site has also had similar moves backfire on them in the sports world, such as:

During the 2009 PGA golf championship, the site paid out early on Tiger Woods, before South Korea's Y.E. Yang overcame a two-stroke deficit on the final day to beat Woods by three strokes.

In 2012, the site paid out early on bets that Manchester United would win the Premiership, only to have cross-town rival Manchester City take the crown.

With the $1 million already written off, a Trump victory would represent the biggest political payout in bookmaking history.