People across the world thought they were watching the perfect fairytale as Princess Diana walked up the aisle at St Paul's Cathedral to meet her prince.
But behind the scenes it was a very different story.
In the lead up to their special day, Diana was becoming increasingly worried about Charles's relationship with his ex Camilla Parker Bowles.
Just days before saying her vows, the then Lady Spencer found a package containing a bracelet in Charles's private secretary Michael Colborne's office.
She demanded to see what was inside of it and discovered the piece was engraved with the letters G and F, which stood for Gladys and Fred - the nicknames Charles and Camilla used for each other.
So when the big day finally came, it's hardly surprising that she felt uneasy when she saw Camilla - who was on the guestlist - sitting in one of the pews at St Paul's Cathedral.
Speaking on the Channel 5 documentary Charles and Camilla: King and Queen in Waiting, royal expert Jenny Bond said: "Diana told me much later in one of our private conversations that she had felt like a lamb to the slaughter as she walked up the aisle, which was very sad.
"I think she knew that things weren't quite right, and when she saw Camilla in the congregation, was immediately uneasy about it."
Charles and Diana tied the knot in front of 2,000 people on July 29, 1981.
Despite the huge guestlist, the bride was only allowed to invite 100 people.
Politicians, important figures and royalty from around the world all made the list, but Diana used her invites to include her friends, family and former colleagues in the celebrations.
Mary Robertson, Diana's former employer, was one of the guests on the future Royal's list.
Speaking on British TV for the first time in Channel 5's new documentary Charles and Diana: The Truth Behind their Wedding, she said: "We learnt later that Diana had only been allowed 100 invitations out of 2,000.
"But I got one. She remembered us."
She speaks fondly of the historical day in 1981.
She said: "St Paul's was lit up. It's never looked so good.
"There was this lovely buzz. Just one of joy and happiness, except for the handful of people who understood what was really going on."
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