The tapes, one source said, were to be listened to when she was no longer there to advise them about their life ahead amongst the Firm.
The late princess was known to be especially close to her sons, and according to a close friend she was inspired by the tapes she made for Andrew Morton’s biography to make personal ones for them to listen to in the future.
After recording the tapes for the publication of ‘Diana: Her True Story’ in 1992, Diana made her own to share “motherly advice” and her “hopes for the future” with William and Harry.
The source told OK: “As incredible as it was, Diana knew she was going to die young.
“She was very attuned to the spirit world and psychic events.
“She told me she made the tapes for the day she wouldn’t be around to help guide her boys.
“That way, she could continue to be a part of their lives, even after she was gone.
“She thought it was quite clever – and loved the idea of being what she called a ‘nagging mum from beyond the grave’. She found it quite amusing.”
The source elaborated on the “sincere” tape she made for William’s future wife and family, saying she “spoke straight from her heart”.
They went on: “I remember bits of it quite well even after all this time.
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She advised that “for your marriage to survive the public glare and the private pressures of royal life, it will take patience, compromise, understanding, trust, loyalty, and, of course, love”.
In an interview in 2017, the Duke of Cambridge explained how he tried to keep his beloved mother’s memory alive for his own children.
He said: “It’s hard because obviously Catherine didn’t know her so she cannot provide that – that level of detail.
“So, I do [when] regularly putting George or Charlotte to bed, talk about her and just try and remind them that there are two grandmothers, there were two grandmothers in their lives, and so it’s important that they know who she was and that she existed.”