In the final countdown to the US election, Americans and foreigners alike are jostling to bet on a shock Donald Trump victory, buoyed by tightening polls and memories of Brexit.

"Since the news of the FBI reopening the investigation, we're seeing five Trump bets for every Hillary (Clinton) one," says Pat Morrow, head oddsmaker at offshore betting site Bovada.lv.

Gambling on any election is banned in the United States but Americans can partake via a plethora of offshore, online sites such as Bovada, Betfair or Betonline.

The FBI director's announcement last Friday that his agents were reviewing Clinton's email scandal coincided with a tightening in the polls that have slashed the Democratic former secretary of state's previously substantial lead.

While most experts predicted a tightening of the polls anyway, betting has seen an even more spectacular uptick.

A futures market run by the University of Iowa for research and teaching purposes saw the probability of a Trump win rise from nine percent about 10 days ago to 40 percent on Monday.

Around 2,300 players, who can each bet $500 maximum, have invested a total of $314,000, says Joyce Berg, Iowa Electronic Markets director.

There is also a lively trade with bookmakers in Britain, where the maverick Republican's odds are also rising in a country rocked by a series of shock political outcomes since Britain voted to leave the European Union.

- Memories of Brexit -

"Trump certainly currently has the momentum and from as long as an 11/2 chance just two weeks back he is now closing up on Hillary almost day by day," says Graham Sharpe, media relations director at William Hill.

The bookmakers, who are listed on the London Stock Exchange, shortened Trump's odds for the eighth time in a fortnight -- this time from 5/2, around 28 percent, to 2/1, 33 percent, he said.

Paddy Power, which has betting shops across Britain and Ireland, said Monday that it had been one-way traffic in the past 48 hours with 91 percent of all bets placed backing a Trump win.

The firm had already paid out $1 million to punters who bet on Clinton, assuming her victory a foregone conclusion.

Now, the company said, the Republican's current odds of 9/4 make his presidency more likely than London having a white Christmas -- generally the stuff of Christmas card fantasy rather than reality, particularly in the days of global warming.

John Mappin, a 51-year-old British businessman who has spent time in America, says he is "more confident than ever" of a Trump victory and stands to win just over 100,000 pounds (more than $120,000) if he does.

"The polls are very inaccurate. It's very, very similar to the Brexit situation that was here in England," said Mappin, who owns a hotel in Tintagel, Cornwall and a local newspaper group in London.

Final polls before the EU referendum in Britain gave the remain camp a substantial edge, yet the June 23 referendum ended with a 52 percent to 48 percent win for the leave campaign, jolting global markets, sending the pound into a tailspin and forcing the prime minister to resign.

- Nearing record bets -

Mappin has bet on Trump around 30 times in the last 16 months at William Hill, putting down the most money right after the businessman threw his hat into the race, sailing down a golden escalator in June 2015.

"I spent a long time in America and I know the Americans very, very well. I understand how fed up they are with bad leadership," Mappin said.

Sharpe says the odds now mirror betting at a similar stage for Brexit. "Some political punters are clearly gambling that the recent trend of shock political results... is going to continue," he said.

Even if the US election race is still highly volatile, which could dissuade some betting, the amounts being taken in are still approaching record highs.

At Bovada it is nearly double the amount bet in the 2012 presidential race between Barack Obama and Mitt Romney, and William Hill is on track to reach three million pounds, which would be a record for a US election.

And a Trump victory won't only be bad news for those betting on Clinton, it would hammer bookmakers.

Paddy Power said it would see the Irish bookmakers face its biggest political payout since in its 28-year history.

"Him winning would be as bad as being on the wrong side of any Super Bowl," agreed Morrow of the most watched sporting and television event in the United States -- the annual NFL final in American football.