A photo of Princess Diana and rock legend David Bowie smiling side by side at a concert in London seemed to have been hidden for decades.

But why did the royal family want to keep it a secret for all these years?

Keep reading to find out.

The year was 1987. Princess Diana was 25 years old and attending a charity concert at Wembley Stadium, where the legendary David Bowie would be performing that evening.

Promoter Harvey Goldsmith arranged the photoshoot, and photographer Denis O’Regan, who had worked with Bowie before, was invited to the scene.

O’Regan now tells the Daily Mail that Diana was suggested a photo together, and she wondered if Bowie would agree to it.

Bowie agreed, and the picture was taken.

However, the next day, the palace issued a request: the photo should not be published.

It wasn’t that the picture was somehow inappropriate. It wasn’t that Bowie was in it. It was about who was with Diana.

The princess had arrived at the concert with Major James Hewitt. There were rumors (that were later found to be true) about an affair between the two at the time, and the palace didn’t want to add any more water to the rumor mill. Hewitt tried to keep a low profile and stayed out of the photographers’ lenses. “No one knew who he was so I had no reason to picture him and the word wasn’t out yet about their relationship,” O’Regan said.

So he decided not to publish the picture himself – and he still stands by his decision.

Diana later said in her biography that she was also criticized for her attire during the evening. She had chosen to wear leather pants – and it made the royal family unhappy.

Diana defended her choice; she wanted to look the same age as her audience.

Now, nearly 40 years later, the image has finally been released as part of a new biography of David Bowie, featuring a wealth of footage taken by photographer Denis O’Regan – and that one moment that was once hidden.

The picture no longer threatens the royal family’s image, but it reminds us of how much behind the scenes there was a desire to control how Princess Diana was presented to the world.