Gay suffered the injury at the US Olympic trials
World 100m champion Tyson Gay has pulled out of his showdown with Jamaica's Asafa Powell at Crystal Palace because of a hamstring injury.
The race, which is due to be held on Friday, was set to be one of the highlights at the London Grand Prix.
American Gay has decided not to risk the injury before the 100m heats begin at the Olympics on Friday 15 August.
"I don't want to risk doing anything to set things back," Gay said. "I apologise to the London fans."
Gay is undergoing treatment for the injury in Germany but is expected to be ready to compete in Beijing as it is said to be healing well.
He suffered the problem during the 200m at US Olympic trials and an MRI scan showed a mild strain but it was later diagnosed as serious enough to visit a specialist in Munich.
The meeting in London on Friday will also feature 100m world record holder Usain Bolt who is scheduled to run in the 200m.
Bolt, who recorded a time of 9.76 in Kingston on 4 May, is seen by some observers as the favourite for Olympic 100m gold.
Top British sprinters Marlon Devonish and Craig Pickering are also expected to compete at the meeting which is being staged for the first time over two days.
The reigning Olympic champion in the women's pole vault, Russian world record holder Yelena Isinbayeva, is also due to compete.