Major Tim Peake, from Westbourne near Emsworth, will be taking on the 26.2 mile distance today on a treadmill aboard the International Space Station (ISS).
The Chichester-based astronaut has recorded a good-luck message, ending with the 10-second countdown, which will be shown to the runners on a big screen ahead of the race start.
He will then join them in running the marathon, starting at 10am. On Earth, a team of UK Space Agency and European Space Agency (ESA) colleagues will run with him, in support of the Prince’s Trust.
Major Peake said: ‘As soon as I was assigned my mission to the International Space Station, I realised that it was a wonderful opportunity to run the Digital Virgin Money London Marathon from on board the ISS.
‘Being an ambassador for the Prince’s Trust, the marathon gives me the perfect opportunity to raise awareness for the important work they do to help disadvantaged young people to get their lives back on track.’
To counteract the effects of weightlessness, he will use a harness to keep him on the running belt.
To donate, visit uk.virginmoneygiving.com/team/teamastronaut
A live stream of Tim’s marathon effort is due to be available from 10am by clicking here