The event, which was originally due to take place on April 26, had been postponed until October 4, but the mass event for amateur runners has been postponed, with 2021's race also moved from April to October.
Elite races for men, women and wheelchair athletes will take place on an enclosed looped course in St James's Park on October 4 in a secure biosphere, with the times counting towards Tokyo 2020 Olympic qualification.
Athletes confirmed for the race include Eliud Kipchoge, Kenenisa Bekele, women's world record holder Brigid Kosgei and para athletes David Weir and Manuela Schar.
No spectators will be allowed at the 40th London Marathon, but BBC Sport will broadcast coverage of the race, encompassing 19.8 laps around the park and finishing in the traditional place on the Mall.
"Today is a day of sadness, but also I think it's a day of certainty," event director Hugh Brasher said, hailing the "inclusivity" of the new-look race.
"It's a day where we're announcing what is appropriate - we think - for the 40th race. It's certainly not something we ever expected to do."
He added: "We hope people will get inspiration from the gods of our sport still battling it out over those 26.2 miles in the only world marathon race that is taking place in the fall.
"And still having Eliud Kipchoge, Kenenisa Bekele, Brigid Kosgei, Manuela Schar, David Weir... they'll take the inspiration from those athletes, on the journey that will be incredibly personal to them. We will celebrate that journey from a distance, but in spirit, together."
Those who had planned to take part in the event - originally postponed from April 26 to October due to the coronavirus pandemic - have been offered the option to defer their places to the 2021, 2022 or 2023 events.
This year was due to be the 40th edition of the race, which has become one the most popular marathons in the world since its inception in 1981, and last year featured more than 42,000 competitors.
The race’s cancellation for amateur runners is a huge blow for charities, with the 2019 renewal raising more than £66milliion.
The decision means that five of the six World Marathon Majors will not take place this year, with Berlin, Chicago, New York and Boston all having cancelled their respective events. Only the Tokyo Marathon, which was held as an Elite-only race in March, has been able to go ahead.
It is also the latest high-profile athletics event to be called off in the UK, with the Great North Run, Anniversary Games and Gateshead Diamond League all having fallen victim to the pandemic.