British police said the trio had done nothing illegal by pulling off an audacious coup at the Ritz using gadgetry to calculate where a roulette ball would land.
The three are to be allowed to keep their $2 million winnings, officers said.
Police had seized the cash and frozen bank accounts during a nine-month investigation into the alleged sting.
The gamblers, described by police sources as a chic and beautiful Hungarian woman, aged 32, and two elegant Serbian men, aged 33 and 38, were alleged to have smuggled into the casino a laser scanner inside a mobile phone that was linked to a micro-computer.
The scanner measured the speed of the ball as it was released by the croupier, identified where it fell and measured the declining orbit of the wheel.
The data was beamed to the micro-computer, which calculated on which section of numbers the ball would land.
This information was then flashed on to the screen of the mobile just before the wheel made its third spin, by which time all bets must be placed.
Having reduced their odds of winning from 37-1 to 6-1, the trio placed bets on all six numbers in the section where the ball would definitely end up.
On the first night they won more than $180,000, returning the next night to win $2 million.
After the casino's security experts later examined closed-circuit television footage, officers from Scotland Yard's gaming squad arrested the trio at a nearby hotel on suspicion of obtaining their winnings by deception.
They were given bail but have now been told they are free to
Legal sources said the gamblers were let off because it was deemed they had not violated any law, since the scanner did not interfere with the ball or wheel, The Sunday Times reported.