Punter Paul Dean will collect at least £250,000 on Thursday after deciding to partially cash out his £5 accumulator.
Dean stood to bank £511,225 if he allowed the bet to roll on and Envoi Allen won the opening Marsh Novices’ Chase.
However, he has opted to take Betfair’s cash-out offer after overnight deliberation.
Dean said: “It’s a fantastic offer and I couldn’t be happier. I’ve definitely done the right thing.
“It’s all about Cheltenham for me and I love having small ante-post bets and the dream of a big win over the winter. I usually start placing my bets at the next best race meeting after Cheltenham – that’s Royal Ascot in June.”
The accumulator kicked off with Golden Horde (12-1) and Hello Youmzain (10-1) winning the Commonwealth Cup and Diamond Jubilee Stakes respectively at Royal Ascot, before Shishkin, who he had backed at 4-1, claimed the Arkle and Bob Olinger, who he had at 25-1, lifted the Ballymore.
Dean said: “This year I really fancied Golden Horde and Hello Youmzain at Royal Ascot. Shishkin was an obvious one, while Bob Olinger looked so good in his bumper.
“I also love his trainer, Henry de Bromhead, and Bob Olinger’s sire, Sholokhov, plus the Ballymore is my favourite race in the world. I’ve backed all the big winners of the race over the years.
“I just love Envoi Allen and want to be able to cheer him home today, that’s why I have partially cashed out. Come on Envoi.”
Asked what he will do with the money, Dean said: “I’ll be going on a lot of holidays!
“It’s my mum and dads 70th this year, so we’ll be going to New York and to some of the best golf courses in the world. I really want to play Bethpage and Sawgrass. And I think I’ll buy a horse.”
Betfair spokesman Barry Orr added: “At Paul’s request, we gave him a partial cash-out offer which reflected his desire to have a portion of his bet still running on Envoi Allen, while also ensuring a substantial profit on his incredible bet.
“The offer sees Paul receive £250,000 if anything other than Envoi Allen wins the Marsh Novice Chase and an additional £50,000 should Envoi Allan win. Giving him a total of £300,000 and fulfilling his wish of being able to cheer home Envoi Allen with gusto.”