The Apache pilot blurted out that the royals had plans to take on the epic 26.2-mile race in 2013 during a live television interview.
He joked: "I have been trying to get myself a place for years. I can’t get myself a place. What do you have to do?
"What's fantastic is my brother and his wife will be doing it next year, I think."
The BBC's Sue Barker replied: "You've just said that live on air."
Harry responded: "He's going to have to do it now, isn't he?"
The royal made his pronouncement as runners tackling the 32nd London Marathon pounded a route that this summer’s Olympians will face.
Up to 37,500 were expected to complete the course today which will take in the home straight of the London 2012 marathon course, leading up the Victoria Embankment to the Mall.
