Sources close to the McCanns have described the picture's emergence as "a very promising lead".
The picture of the blonde girl was taken on 31 August, nearly four months after she went missing.The woman who took a photograph is in no doubt the photo shows Madeleine. Clara Torres said today: "Either it's her or she's got a twin."
The McCanns are waiting for face-mapping experts at the Child Exploitation and Online Protection Centre to tell them if it is their daughter. It could be the first step towards solving the mystery of Madeleine's disappearance from her parents' holiday apartment in Portugal on 3 May.
Mr Mitchell said the picture posed "serious questions", adding: "There is a chance it is her, but we have to exercise caution." It is the fourth possible sighting of Madeleine in Morocco since she vanished.
Ms Torres took the photograph which shows the child being carried on the back of a woman who is accompanied by several adults of North African appearance as they walk along the side of a road.
She said: "I was with my family and my partner, we were going from Chaouen to Tetuan. On the road, in a town called Zinat, I was taking photos of everything we saw, and we saw a group of people and took a photo. As soon as I took the photo we were struck by the little girl, who was very blonde. I was on a trip in Morocco and took many pictures, just random ones. This was one of those pictures. I remember being struck by her and even mentioned the name Madeleine to my boyfriend."