Prince Charles' secret note to Diana on the eve of their Royal Wedding

Prince Charles wrote his bride-to-be Diana Spencer a sweet message on the eve of their wedding, despite reportedly having doubts about his marriage, an insider has claimed.

The heir to the throne, who was 32 as he prepared to tie the knot, sent the note and gift to Clarence House where Diana spent the night.

Despite reportedly having doubts about their marriage, the note was supportive and offered her advice for the big day.

In her new book about Camilla, royal author Penny Junor writes: "The night before the wedding, which Diana spent at Clarence House with her sister Jane, he sent her a note, along with a signet ring that bore the Prince of Wales feathers.

"He wrote: "I'm so proud of you and when you come up I'll be there at the altar for you tomorrow. Just look 'em in the eye and knock 'em dead'", reports the Express.

Ms Junor also claims Charles believed the wedding was a mistake but knew it was too late to pull out.

She claims Diana changed in the lead up to their wedding and she became moody and tearful.

She writes: "Charles was not convinced he was doing the right thing in marrying Diana but there was no way out and, bolstered by the hope that things would be different once they were married, he put a brave face on it."

Charles and Diana married in a lavish ceremony on July 29, 1981 which was watched by 750 million people around the world.

Earlier this year royal reporter Robert Jobson claimed Prince Charles knew the wedding was a "massive mistake" before he said I Do in a new biography.

Charles is said to have told his friends: "I desperately wanted to get out of the wedding in 1981, when during the engagement I discovered just how awful the prospects were, having had no chance whatsoever to get to know Diana beforehand."

Before starting his relationship with Diana he dated Camilla, but she was not considered a suitable match.