Paula Radcliffe has withdrawn from the London Marathon on 13 April because of a toe problem.
The 34-year-old Briton, who was aiming to win her fourth title, picked up the injury to her right foot when altitude training at her base in New Mexico.
"I am desperately disappointed that I have to pull out of this year's race," said Radcliffe.
"I love running in London and this race would have been the perfect test for me before the Olympic Games."
The current women's marathon world record-holder won the London event in 2002, 2003 and 2005.
Radcliffe described the injury as "minor" but wanted to be in the best shape for London.
"I must concentrate on being as well prepared as possible for Beijing"
"In marathon training there are no shortcuts and there is simply not enough time to be in the shape I want to be in to run well in London," she added.
"Now I must concentrate on being as well prepared as possible for Beijing."
BBC Radio 5 Live's athletics correspondent Mike Costello said Radcliffe had been serious about competing at this year's London showpiece.
"It was a surprise to some she even considered running in the marathon in this Olympic year - but she felt it would play an important part in her build up to Beijing," he stated.
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"She also missed London in 2004 when her preparations before the Athens Games included track races over 5000m and 10,000m."
Race director David Bedford said he understood Radcliffe's reasons for pulling out.
"Of course, it is a major blow to lose Paula from this year's race but we realise that her priority in 2008 is to be in peak condition for the Olympic Games and we wish her all the best for a speedy return to full fitness," he said.
"Even without Paula we will still have a very strong international women's field on 13 April and I am sure the race will be as exciting as ever."
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