43-year-old Tim Letten, from Essex, was taking a picture of the famous London landmark, when the mysterious orb was captured along in the photo.
He, however, did not think much of the object until he uploaded his photos on his home computer.
"I was taking pictures along the river for a competition and captured a picture of an object flying across the sky which I thought was an aeroplane," the Sun quoted him as saying.
"When I got home I put it on my computer, zoomed in and thought 'It must be a UFO!'
"Looking at that, if anyone doesn't believe, they must be mad!" the excited designer added.