A spokesman for the Royal Sussex County Hospital said the man died at 2.30pm.
England cricketer Matt Prior had officially started today's race, the fourth Brighton marathon, in Preston Park at 9am.
Organisers said 9,315 competitors were taking part.
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Eurostar passengers were left stranded last night after migrants reportedly climbed on top of trains at Calais.
Rogue landlords are making life unbearable for tens of thousands of tenants in England, according to Shelter.
Wednesday's weather for the west of the region