The Queen was so “deeply concerned” about Diana as her marriage to Prince Charles unravelled that she took a close friend aside to check on her emotional state.
Harry Herbert, whose father was the 7th Earl of Carnarvon and racing manager to the Queen, spoke to the Monarch at Balmoral in 1992.
It was in the weeks leading up to Diana and Charles separating.
Speaking to the ITV Documentary, Diana, Our Mother: Her Life and Legacy, Mr Herbert said: “That was a bad time for Diana. You know the light had gone out if you like.
“I had a talk to the Queen about it at Balmoral. The Queen wanted to talk to me about it because she was so worried about Diana.”
The pair spoke after a lunch at the castle in Aberdeenshire and the Queen wanted to know “how was Diana feeling”.
Diana the AIDS campaigner: Prince Harry hears tribute from Elton John
Mr Herbert describes it as “a sad discussion” because “that was everything at its worst”.
He then went to visit Diana at Kensington Palace.
But said that “although things weren’t particularly easy” in her married life that her young sons, Princes William and Harry, could still bring a smile to her face.
He said: “She was, you know, very emotional. And, suddenly, these two boys came thundering round the corner in their dressing-gowns, this was before bed, and just watching her face light up.
“Going from sad chat to suddenly, boof. You know, I’ll never forget that moment, and them, you know, crawling all over her and things flying everywhere.
“And through all the difficulty of other stuff at that time, you could see it was the most important thing in her life were her boys.”