Penny Thornton, who Diana started consulting with in 1986, claimed the statement made her want to call off the wedding in 1986.
Speaking on new ITV documentary The Diana Interview: Revenge of a Princess, she said: “One of the most shocking things that Diana told me was that the night before the wedding, Charles told her that he didn’t love her.
“And that was a truly terrible thing to do to your future bride that you’re going to marry in St Paul’s the very next day.
“I think Charles didn’t want to go into the wedding on a false premise, he wanted to square it with her.”
Penny said the four words left Diana devastated and almost saw her put an end to the wedding.
“She didn’t want to go through with the wedding at that point, she thought about not attending the wedding,” she added.
Elsewhere in the documentary, Charles was branded “atrocious” by royal photographer Ken Lennox for leaving to play Polo minutes after Diana gave birth to their son Prince Harry.
Mr Lennox, who was waiting outside St Mary’s Hospital in London to take pictures of the couple and their newborn son, said he found the rumor so ridiculous he bet £50 it wasn’t true – and lost.
Ken said in the show: "I lost £50 because I couldn’t believe that a man with a newborn baby was going to leg it and play some daft game of Polo.
"Doing something like that to a wife with a newborn baby was atrocious.
"If he thought he could treat his wife that, he was in deep trouble."