The 25-1 shot beat second favourite Strong Suit by a neck, with Adam Kirby's superb ride seeing him soar into an early lead and then hold off all challenges.
Alanza came in third at 9-1, while the 2-1 favourite Soul, ridden by Frankie Dettori, finished out of the places down in sixth.
"We thought we'd fit the blinkers and change the tactics - and it worked! We've always rated him and knew he was capable of a performance like that," said Lethal Force's trainer Clive Cox.
"After he was fourth at Royal Ascot the owners turned down a big offer from overseas and thank goodness they did. He may run in the Park Stakes."
In the day's other big races at Newbury, Just the Judge won the 2:00pm Denford Stud Stakes at 3-1 ahead of Excess Knowledge (11-4), while 5-4 favourite Mount Athos won the Betfred "The Bonus King" Geoffrey Freer Stakes at 2:30pm ahead of 9-4 Brown Panther.