Matt Campbell fell at the 22.5-mile mark and was given medical treatment from race doctors but died later in hospital.
His family paid tribute to him as an “inspirational son and brother”.
Mr Campbell, who reached the semi-finals of MasterChef: The Professionals last year, was running the marathon in memory of his father, Martin.
In a statement, race organisers said: “Everyone involved in the organisation of the London Marathon would like to express our sincere condolences to Matt’s family and friends.
“No further details will be released and the family has asked for privacy.
“The exact cause of death will be established by later medical examination.”
Sunday’s London Marathon was the hottest on record, with temperatures reaching 23.2C (73.8F).
Food writer Jay Rayner paid tribute to Mr Campbell, calling him “hugely creative and innovative”.
Restaurant critic Grace Dent added that Mr Campbell was “brilliant, clever, funny” and “incredibly talented”.
A spokeswoman for MasterChef said: “We are shocked and saddened to hear the news about Matt Campbell, one of our talented contestants from last year.
“It was a privilege to have him on the show. He will always be remembered for producing for some of the most innovative and groundbreaking food that we saw on the series.
“From the whole MasterChef team, our sincere condolences and thoughts are with his family and friends.”
Mr Campbell, a keen runner from the Lake District, had completed the Manchester Marathon earlier this month in under three hours.
According to a JustGiving page, Mr Campbell had surpassed his target of raising £2,500 for youth charity The Brathay Trust by competing in the London event.
Donations quickly soared after the professional chef’s death was announced, with thousands more pounds pledged.
On the site, Mr Campbell wrote that his late father Martin had worked “tirelessly” for The Brathay Trust, describing him as “the most inspirational man in my life”.
Mr Campbell had posted: “The past year and a half have been the toughest of my life but his spirit and energy live on in me.”
He added that his father had entered him into his first marathon in 2016, along with his brother Josh.
“It was maybe the proudest day of my life standing at the finish line having achieved something I never imagined possible with my Dad & my Brother by my side,” Mr Campbell wrote.
According to a biography on his website, Mr Campbell began his career working in Michelin-starred kitchens after finishing second on BBC Young Chef of the Year at the age of 20 in 2009.
Following his successful stint on MasterChef: The Professionals in 2017, he embarked on a UK roadshow this year – with dates in London, Manchester, Bristol and Leeds.