Record pounds 21.8m jackpot for cocktail waitress.
Three Megabuck symbols came up on the machine in a coffee shop at the Desert Inn - winning Ms Cindy Jay a world record pounds 21.8 million.
The 38-year-old had invested pounds 17 in the machine before she struck lucky and was only gambling because she had failed to get tickets for a show she wanted to see.
Mr Ed Rogich, vice president of Megabucks maker International Game Technology, said: "Right now she's trying to gather her thoughts and discuss things with her family."
The official amount of the jackpot was $34,955,489.56.
Ms Jay was given $1,402,188.58 (pounds 876,000) immediately and has 90 days to decide whether to take payments in annual instalments over the next 25 years or receive a lump sum of about 60 per cent of the total - pounds 13 million - before taxes.
But whatever Ms Jay decides she will be hearing from the US tax man - who will demand almost 40 per cent of her winnings.
The Internal Revenue Service will treat the jackpot as ordinary income subject to the maximum tax rate of 39.6 per cent.
"We are absolutely thrilled the Desert Inn is now home to the world's largest slot jackpot," said Mr Mark Lefever, chief operating officer of the resort. "As we prepare for our 50th anniversary in April, it is only appropriate that the newest Megabucks millionaire be christened at this historic property."
Wednesday's jackpot topped the previous world record of pounds 17.25 million, won in 1998, by a 67-year-old Las Vegas woman who requested anonymity.
The main Megabucks jackpot now has been hit 49 times, paying off more than pounds 188 million. The game builds from a base amount of pounds 4.3 million, with each spin adding a few pennies to the top jackpot.
The odds against hitting the top prize are about 49.8 million to 1.