BEIJING — Defending Olympic champion Mizuki Noguchi pulled out of Sunday's marathon because of a lingering left thigh injury.

The Japanese delegation announced the diminutive runner's withdrawal Tuesday.

Noguchi, the 2003 world championship silver medalist, had hoped to become the first woman to repeat as Olympic marathon champion.

She had been bothered by the injury for some time and underwent a series of tests in Japan after returning from her training in St. Moritz, Switzerland. Noguchi, who turned 30 last month, pulled out of last year's world championships because she doesn't like running in the heat.

However, she survived sweltering conditions to win the event at the 2004 Athens Olympics, beating favored Catherine Ndereba of Kenya by 12 seconds. She won the Tokyo Marathon last November.

Paula Radcliffe, the world record holder from Britain, was a major disappointment in that race, collapsing because of dehydration after leading much of it.

The 34-year-old Radcliffe told the BBC on Tuesday that she would race in the marathon in Beijing. She too has been bothered by a thigh injury.

"Of course, I could do with a bit more time, but I'll just go in and give it a go," Radcliffe said.

Noguchi has been plagued by injuries since her Olympic triumph. Her lone race in three years came last November when she won the Tokyo Marathon.

Two months ago, she spoke confidently of her prospects and talked about wearing ultra-light shoes partly made of rice husks.

"The shoes are easy to run in," Noguchi said at the time. "The Beijing course is hard on the feet and legs. It's flat and the surface of the roads is very hard."

She also spoke of trying to become the first repeat winner in the women's marathon, which was added to the Olympic program in 1984.

"I don't really feel pressure," she said. "To the contrary, it makes me happy that I'm getting this much attention."

In April, Noguchi had carried the Olympic torch through Nagano, site of the 1998 Winter Games, protected by riot police from anti-China protests.

"As I ran, I wished for the success of the Beijing Olympics and peace," Noguchi said.