Matt Campbell went down 22.5 miles into the race and was treated by doctors before being rushed to hospital.
Campbell, who completed the Manchester Marathon this month in under three hours, was running in memory of his father Martin who died two years ago.
"Everyone involved in the organisation of the London Marathon would like to express our sincere condolences to Matt's family and friends," organisers said in a statement.
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"The exact cause of death will be established by later medical examination."
Campbell last year reached the semi-finals of the BBC television show "Masterchef: The Professionals".
(Reporting by Shrivathsa Sridhar in Bengaluru. Editing by Ed Osmond)
