Olympic sprint champion Usain Bolt has fallen to another defeat on the eve of his historic London Olympics mission.

Bolt, who is aiming to be the first Olympian to win back-to-back titles in both the 100m and 200m events, has been beaten in both races at Jamaica's Olympic trials.

Training partner Yohan Blake edged the superstar on each occasion, triggering speculation that Bolt could yet surrender his title as the world's fastest man. (AAP Images)