The 2020 London Marathon will involve only elite athletes, with 45,000 'mass-event' runners unable to take part because of coronavirus concerns.
The much-anticipated contest between Kenya's Eliud Kipchoge and Ethiopian Kenenisa Bekele was due to take place on a bio-secure closed course.
Reduced fields of 30-40 athletes will also compete for the elite women's and wheelchair titles.
The 2021 race, meanwhile, will be on October 3 rather than a date in April.
That calendar shift from the traditional date is designed to maximise the chances of all runners being able to take part in next year's race.
A plan to include the mass-participation event in the 2020 race, deploying high-tech tracking technology to monitor runners' proximity to each other, had been considered.
However, event director Hugh Brasher said that plan had been made impossible by the logistical challenges of managing spectators and emergency service access across London, especially given the recent cancellation of spectator trials at other sporting events.
Instead, it will only be the elite athletes that tackle a spectator-free course - following a different route to the usual one used for the London Marathon.
That route will consist of laps of roughly 1.5 miles, taking in The Mall, Horse Guards Parade, Birdcage Walk and the spur road running adjacent to front of Buckingham Palace.
The showdown between four-time winner Kipchoge and Bekele, whose personal best is two seconds slower than Kipchoge's world record of two hours one minute 39 seconds, has now been put on the back burner after the Ethiopian had to pull out through injury.
British Paralympic great David Weir will be aiming for a record ninth win in the wheelchair race.
Kenya's defending champion Brigid Kosgei, who beat Briton Paula Radcliffe's long-standing world record in Chicago in October, will headline the women's field, with course record holder Manuela Schar attempting to follow up her 2019 win in the women's wheelchair race.
Athletes' times in the race will be eligible for Olympic qualification for the postponed Tokyo Games in 2021.
While this year's Tokyo marathon took place in a similar form in March, with only elite runners taking part and spectators restricted in number, other major marathons have been cancelled.
The Berlin and New York races, which were scheduled to be held on September 27 and November 1 respectively, are among those that will not take place in 2020.
Runners with a place in the 2020 race, but not in the elite fields, will be able to compete virtually from any location around the world.
They are invited to run or walk 26.2 miles, taking breaks if required, over the course of 24 hours, logging their progress on the event app.
Last year, the London Marathon raised £66.4m for charities and good causes.
Brasher said: "We believe that tomorrow will see a London Marathon like no other, taking the spirit of the world's greatest marathon to every corner of the globe, with runners raising vital funds for the charities that have been so severely affected by the economic effects of the pandemic."
