The runner, named in reports as Claire Squires from Leicestershire, collapsed at Birdcage Walk near St James’s Park, within yards of Buckingham Palace and the final turn into The Mall which marks the final stretch of the 26.2 mile race.
She was given medical attention at the scene by paramedics but later died, organisers confirmed.
A statement on the London Marathon website said: "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.
"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."
The death is the 10th in the race since 1981. Five of those were runners who took part in the race unaware that they had heart conditions.
The last competitor to die was a 22-year-old fitness instructor in 2007.
Up to 37,500 runners ran the 32nd London Marathon, with Prince Harry among the vast crowds of spectators.
Kenya’s Wilson Kipsang won the men's race with a time of two hours four minutes and 44 seconds while his compatriot Mary Keitany won her second consecutive London Marathon title with a time of two hours 18 minutes and 36 seconds.
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