Harry, 69, who has retired from his Costcutter business in Macclesfield, Cheshire, will pound the capital's streets in aid of The Multiple Sclerosis Society.
He said: "My sister-in-law suffers from MS so it's a charity which is close to my heart."
Harry is no stranger to the distance, having completed the London marathon six times. In 2006 he ran the course in three hours 45 minutes and is hoping for a similar time this year.
Anyone who would like to sponsor Harry can visit www.justgiving.com/harrynewton to make a pledge.