Britain's 9,000 betting shops are planning to bring in an estimated 10,000 machines by Thursday.
The new law also increases the amount of cash that can be paid out to pounds 10 but makes it illegal for under-18s to play.
On average, a single fruit machine makes pounds 10,000 profit each year. Ladbrokes, Britain's biggest bookmaker, says it plans to have two machines, the maximum allowed, "in every shop that can physically take them".
Ladbrokes has 1,900 shops and is hoping to see profits of around pounds 6m in the first year.
City analysts say it is hard to know how much the bookies will make on fruit machines, but the "golden scenario" for a company like Ladbrokes, with so many outlets, would be profits of pounds 38m. A more likely estimate is between pounds 5m and pounds 10m, and less for the smaller betting-shop operators Coral and William Hill.
In Ladbrokes in Clapham High Street on Friday, the fruit machines sat waiting in the corner, wrapped up in paper with big question-marks on. But punters were disapproving. "It's a terrible idea," said Paul, who was there for a quick flutter on the dogs. "It will be disastrous in a betting shop!"
Another customer complained: "They are being greedy enough as it is. They are already milking us on the odds. What's the point, when you can put pounds 1 each way on a 12-1 shot and get about pounds 16 back. You see people putting pounds 75 in and walking away with pounds 6."
Others were worried that the machines will be too noisy when they are trying to study form.
But bookmakers have been lobbying hard for change in the gaming laws, arguing that they have been badly hit by the National Lottery, particularly since the introduction of scratch cards in March. Ladbrokes says it has lost 10 per cent of its bets.
But Dr Emmanuel Moran, a psychiatrist who specialises in gambling addiction and is chairman of the National Council on Gambling, says the evidence that bookmakers have lost out to the National Lottery is mixed. "Anyway," he says, "it's a curious argument that because people spend more money on a new form of gambling we should encourage them to spend even more on the old form. Even before the lottery there was more gambling in Britain than in any other European country."
The British already put pounds 9bn into fruit machines every year. The "fruity" is the nation's second favourite form of gambling, after the National Lottery.Reuse content