The north Londoner, who did not want to be named, placed £2 worth of bets covering seven horses running at Lingfield, Doncaster and Newbury on Friday.
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Ladbrokes bookmakers said it was when the horse Mcvicar beat the hot favourite in the 5.05pm at Doncaster that six of his picks had won.
It meant he scooped £457,067.52 from a 10 pence each way six-fold accumulator which had been placed as part of the multiple bet.
The lifelong racing fan had not bothered to check all the results after one of his picks called Bendomingo lost a race at Newbury.
He said he was "over the moon", adding: "I checked a couple of the early results on my tablet but then after seeing one of them had lost I thought I might have only been on for a couple of places, so didn't bother to see how the rest of the horses had got on.
"The next thing I knew, someone from Ladbrokes was phoning me to congratulate me for winning nearly half a million quid and to ask how I'd like the money, and I had no idea what he was talking about.
"I was completely speechless and I think it's going to take a few days to sink in properly."
His winning bets were on Monfass and Mcvicar, who were both on at odds of 12/1, Dinsdale who was on at 6/1, Peal Spectre at 10/1, Boater at 15/2 and Wishicould at 14/1.
He also bet on Bendomingo who lost.
His payout was boosted by an extra £228,000 due to Ladbrokes Best Odds Guaranteed concession.
Alex Donohue of Ladbrokes said: "We're sure this is one bet our customer won't be able to forget now and we couldn't be happier to pay the winnings to a lifelong racing fan.
"We're sure from now he'll follow racing results more closely as they come in."