The woman had collapsed near to the end of the 2012 marathon, with the finishing line only one bend away.
Birdcage Walk borders St James’s Park and is the last road that runners have to travel before reaching Buckingham Palace where they turn onto The Mall on which the finish line is located.
Competitors do not reach Birdcage Walk until after completing 25 miles of the marathon.
In a statement the organisers said: “It is with regret that we have learnt of the death of a competitor in the Virgin London Marathon.
“A 30-year-old woman collapsed at Birdcage Walk, and although immediate medical attention was provided to the casualty, the fatality was confirmed this afternoon.
“The organisers of the Virgin London Marathon would like to express their sincere condolences to the family and friends of the deceased.
“We will not be releasing any further details of this tragic incident until next of kin have been notified and formal identification has taken place. We would ask for your understanding in this matter.
“We would like to emphasise that our immediate concern is for the family of the deceased. Our thoughts and deepest sympathies are with them at this difficult time.”
The death is the first during the race since 2007, when a 22-year-old man was taken ill and died. In total 10 people have died during the race since it began in 1981.
Five of the previous fatalities were a result of heart disease in runners apparently unaware that they had a problem. Four of these were cases of severe coronary heart disease.
According to sports website Peak Performance the incidence of death in the London marathon is rare – equivalent, it says, to one in 67,414, or roughly one death for every two million miles run.
Up to 37,500 runners set off in bright sunshine through the streets of the capital to earn their medals and raise money for countless charities.