Most of the money wagered on Sunday's Super Bowl will be through illegal channels, due to a near-blanket ban on full-scale betting in the US.
Some $4.6bn (£3.2bn) or 97 per cent of bets will be made through sports books that are not licensed in Nevada.
The US state is the only one which is exempted from a federal ban on full-scale sports betting, meaning US bets placed outside of Nevada are illegal.
But UK punters are free to take a bet on the major event, as the ban only applies to people in US jurisdictions other than Nevada.
Only three per cent, or $138.5m, of the total gambling outlay is expected to be wagered through licensed sports books, the American Gaming Association has said.
London-listed bookie William Hill has 108 locations in Nevada and said it can offer legitimate betting to the state's residents and visitors.
Two customers have already put huge bets of more than $1m on underdogs the Philadelphia Eagles to win, but the New England Patriots are still the favourites to win. Patriots quarterback Tom Brady is the favourite to be Most Valuable Player.