Kent Gavin, a royal photographer, was with Princess Diana on her wedding day and the day before.

He said she was upset the day before the wedding and left an event.

"It was a very happy occasion for everybody apart from Diana, sadly," Gavin said of the wedding day.

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It's been 40 years since Princess Diana and Prince Charles' wedding.

Their marriage was celebrated worldwide, with over 750 million people tuning in to watch Lady Diana Spencer wed the future king of England.

The wedding was heralded as a celebratory day for everyone - except Diana, according to Kent Gavin.

Gavin worked as the royal photographer for the Daily Mirror, and he documented her wedding day.

He said he was with Diana the day before her wedding, as he had been given rare direct access to the royal family.

Gavin told Insider that she was visibly unhappy the day before she said "I do.

"The day before, I was with her at Smith's Lawn," he said. "Prince Charles was playing polo, and she was there.

"She was very upset and left," he went on.

Later, Diana revealed that she'd had doubts throughout her engagement about marrying the prince.

As Insider's Mikhaila Friel reported, Diana said in a series of tapes recorded in 1991 that she became doubtful when she discovered Charles had a bracelet made for Camilla Parker-Bowles, now the Duchess of Cornwall.

The princess said that she confessed those doubts to her sisters but that they told her to go on with the event. She said they told her, "Your face is on the tea towels, so you're too late to chicken out.

In the tapes, Diana said her wedding day was one of the worst days of her life.

On the day of the wedding, Gavin was inside St. Paul's Cathedral, so he saw Diana and the rest of the royal family enter.

"The Queen, who I could see quite clearly from my position, was very happy," he told Insider.

"It was a very happy occasion for everybody apart from Diana, sadly," Gavin said.

Gavin developed a close relationship with the princess in the years after the wedding. He traveled with her around the world, and she even chose him to be the photographer for Prince William's christening.

Gavin was with her as her marriage to Charles imploded, even discussing their relationship with him, Gavin said.

"On a trip coming back from Pakistan, we got to talking about the marriage ending," he told Insider. "She did say, 'Kent, I still love him.'

Gavin reflected on his experience documenting the nuptials in BritBox's special "Wedding of the Century," which became available to stream on Thursday.