File photo: The royal couple's separation was officially announced in December that year. Picture: AP

London - The Queen summoned a friend of Princess Diana to ask about the breakdown of her marriage to Prince Charles.

Harry Herbert revealed the monarch was worried about Diana, following months of speculation about the state of her marriage to the Prince of Wales.

The son of the Earl of Carnarvon, who was then the Queen's horse racing manager, was a trusted friend of the princess and said she was going through "a bad time … the light had gone out".

He told the documentary that the Queen had raised the issue of Charles and Diana's marriage after a lunch at Balmoral.

"The Queen wanted to talk to me about it because she was so worried about Diana," he said.

"[We had] an incredibly important chat, a very personal chat. And the Queen wanted to know how was Diana feeling, and was it as bad as … as it was."

He added: "It was a sad discussion, a sad moment really because that was everything at its worst."

The meeting took place before Charles and Diana's official visit to South Korea in November 1992, when photographs revealed their apparent discomfort in each other's company. 

The royal couple's separation was officially announced in December that year.