Harry, 27, let the ‘news’ slip during a TV interview at yesterday’s race.
Harry revealed: “What’s fantastic is that my brother and his wife will be doing it next year, I think.”
BBC presenter Sue Barker, 56, replied: “You’ve just said that live on air!”
The Prince replied: “He’s going to have to now, isn’t he?”
Clarence House then issued a statement saying Harry was only joking and the couple were unsure of their plans at this time next year.
Race organisers said yesterday tough security measures would be put in place if Prince William, 29, and Princess Kate, 30, did decide to take part.
A spokeswoman said: “The London Marathon has strict security arrangements in place for any high profile runners.”
“If William and Kate do decide to run, then these would be put into operation.”