Prince William and Prince Harry last night renewed their calls to Channel 4 not to use photographs of the car crash that killed their mother as the plans had left them "deeply distressed".
Jamie Lowther-Pinkerton, the princes' private secretary, spoke of the emotional hurt screening the images would cause the brothers as the broadcaster stood firm over its decision to show the controversial pictures in the programme being aired tonight.
In a television interview, Mr Lowther-Pinkerton appealed to the documentary makers to think again.
He said the princes had decided "long ago that one of the great duties of their life is going to be to protect their mother's memory, and this I'm afraid clearly infringes that".
Asked why the princes could not ignore the programme as other royals have ignored allegations made against them, Mr Lowther-Pinkerton replied: "These princes are protecting their mother's memory, it's a different context entirely, and they are deeply distressed by it and have asked me to tell you that."
The images to be screened include one of Diana receiving oxygen from a French doctor as she lies dying, but her face is obscured.
Other pictures include the wrecked Mercedes and a view through the back of an ambulance in which the princess was treated.
Mr Lowther-Pinkerton, who wrote to Channel 4 appealing on the behalf of the princes, said: "It is their mother's last moments on earth and it's an invasion of her privacy. We are not objecting to the documentary, we are objecting just to these photographs.
"The photographs tell a story, the story could be equally well told by talking heads, the doctor himself describes the scene very, very vividly."
In his letter to Hamish Mykura, the Channel 4 executive who commissioned the programme, Mr Lowther-Pinkerton asked: "If it were your or my mother dying in that tunnel, would we want the scene broadcast to the nation?"
However, Mr Mykura, head of history, science and religion, revealed that his own father died in a car crash.
He said he would not want images in which viewers could actually see his father dying to be broadcast. But he would not object to images like those in the Diana documentary being shown if it were in the public interest.
"I lost a parent in a road accident so I am in no doubt about the pain that can cause," he said. "I would not be happy for images of my own father dying to be used in a broadcast and that is one of the reasons why we will not be using images of the princess or any of the other occupants of the car.
"But in a case where it is a major international news story in which the factor in dispute is the crash itself, I think there are circumstances in which it would be correct for images of the scene to be broadcast, as long as it was handled in a measured and responsible way."