Among the crowds queuing to sign the books of condolence on Wednesday, a number emerged convinced that they had seen Diana's face in the top right-hand corner of Edward Bower's painting, which hangs in a corridor of the palace.
Oliver James, a clinical psychologist, said it was an extreme manifestation of a recognised phenomenon among the recently bereaved. "One response in some people is to see the dead person all around them.
"They smell them, hear them, sense them everywhere. In extreme cases, people have hallucinations, and it can be very infectious."
Dr Anthony Taylor, a lecturer at Warwick University, added: "People are starting to see Diana as a secular Virgin Mary. In Catholic countries, the Virgin Mary has appeared to people at times of crisis."