A 45-year-old man died while travelling home after taking part in Sunday's London Marathon, race organisers said in a statement.
Steve Shanks, from Nottingham, was an experienced runner who finished the race in two hours, 53 minutes, 26 seconds, organisers said on Wednesday, adding that the cause of his death would be established later through medical examination.
"Everyone at London Marathon Events was deeply saddened to hear about the sudden death of Steve Shanks," the organisers said.
"A fundraising page for the Multiple Sclerosis Society has been set up in Steve's memory."
Kenya's Kelvin Kiptum won the men's race in a course record two hours, one minute, 25 seconds, with Dutch middle-distance athlete Sifan Hassan winning the women's race in her marathon debut in a stunning upset.
More than 48 000 people finished the race.