A Leicester City fan stands to earn £25,000 if his team win the Premier League title after placing a £5 pre-season bet on the Foxes at 5,000-1.
Leicester's 3-1 win at Manchester City leaves them five points clear at the top with two thirds of the season gone.
Carpenter Leigh Herbert, 38, placed his speculative wager while on a camping holiday and has declined an offer to cash in for a guaranteed £3,200.
"Even if they tail off, it's been worth that £5," Herbert told BBC Sport.
If Leicester - who were bottom of the table a year ago - do win the title, it is thought this would be the biggest-priced single winner in sporting history.
Listen: Story behind dream wager (BBC World Service)
How the wager came about
Herbert has been a Leicester fan since the age of 10, when he went to a game with his uncle Adrian, who once played junior football with Foxes legend Gary Lineker.
After a few drinks one night on holiday in Newquay last summer, he placed the bet on a whim following the appointment of Italian Claudio Ranieri as manager.
"I watched them last season and they didn't play that badly. When he was appointed I thought he might bring something," said Herbert, who sold raffle tickets for the club when he was a teenager.
"The first thing I thought the next day was that I'd lost that fiver. I'm not a betting man, apart from a pound or two on the Grand National."
How Leicester's odds have plunged: (Price at start of month) August: 5,000-1; September: 1,500-1; October: 1,500-1; November: 500-1; December: 66-1; January: 16-1; February: 7-1.
|Against the odds - big winners|
|2010: Africa Cup of Nations, 1,000-1. Punter won £5,000 after a £5 bet on Mali to draw with hosts Angola. Mali were trailing 4-0 when the bet was made but drew 4-4.|
|2004: Football, 150-1. Outsiders Greece shocked the football world by winning the European Championship.|
|1996: Horse racing, 25,000-1. Gambler Darren Yates won £550,000 after Frankie Dettori won seven races in a day.|
|1990: Boxing, 42-1. Buster Douglas shocked Mike Tyson to win the world heavyweight championship.|
|1981: Cricket 500-1. England, inspired by Ian Botham, came from behind to beat Australia and win the Ashes.|
Bookmaker faces £2m payout
Herbert was one of just 12 people who backed Leicester at 5,000-1 online with bookmakers William Hill, who say their biggest payout would be to a customer from Guildford who placed £75 each way at 1,500-1 and could collect £140,775.
"For commercial reasons we are not Leicester fans," said spokesman Rupert Adams.
"Should they win the Premier League, we will be paying out £2m-plus. That said, we will not begrudge Leigh as this would be a fantastic feat."
Support from other fans
The Leicester success story has seen them cheered on by some supporters of other teams as the underdogs battle with Manchester City, Arsenal and Tottenham for the title.
"If I'd have spent £5 trying to find as much excitement, I couldn't have got it," added Herbert.
"The majority of messages I've had have been supportive and said it's a great story. I even had a Nottingham Forest fan wishing me all the best."
He is determined to let the bet stand, even though he could walk away with thousands now by 'cashing out' for a smaller amount than his potential winnings.
"I'm trying not to think about missing out on the money. To have them finish in the top four would be out of this world," he said.
"I guess you'd usually bet on the other team as Leicester have disappointed a bit in the past."
What would he do with the money?
Clearing debts and funding a good wedding are top of the list if his bet comes in.
"I'm engaged to my fiancee Kerry and it would be lovely to have a deposit on a house. I've only ever rented," he said.
|Leicester's remaining fixtures|
|Arsenal (14 February)|
|Norwich (27 February)|
|West Brom (1 March)|
|Watford (5 March)|
|Newcastle (14 March)|
|Crystal Palace (19 March)|
|Southampton (2 April)|
|Sunderland (9 April)|
|West Ham (16 April)|
|Swansea (23 April)|
|Man Utd (30 April)|
|Everton (7 May)|
|Chelsea (15 May)|