Princess Diana's favourite breakfast has been revealed by her former personal chef

He said in a YouTube video: "She was now eating healthy?- her go to breakfast was something called overnight oats."

Darren, 65, who served as a royal cook for Diana, the mother of Prince William and Prince Harry, and also explained the princess first discovered the meal on a trip to Switzerland during the early 1990s. He described how Diana began consuming the nutritious mix of oats soaked overnight in milk or juice, often with added fruits and nuts.

He said: "She was looking the best she ever did (during that period.)"

Overnight oats, originally known as bircher muesli, were created by Swiss nutritionist Dr?Maximilian?Oskar?Bircher Benner in the early 20th century to provide patients with a healthy, convenient dish.

Darren emphasised Diana brought the concept to Britain decades before it became mainstream.

He added: "Now today overnight oats is popular. Everybody is eating them but they date back to 2012... but Princess Diana was eating overnight oats almost 20 years earlier in 1993."

Darren described the recipe he prepared for Diana as using rolled oats - not "quick oats" - which he said would be steeped overnight in fresh juice such as orange, then mixed in yoghurt for protein.

A drizzle of raw honey was added for sweetness, and he would stir in blueberries, chopped walnuts, honey crisp apple, lemon juice and optionally chia seeds or other berries.

Darren went on: "The rolled oats in my opinion are the best.

"With the raw honey just do that to taste... If you like it really, really sweet then obviously put a little bit more in but not too much."

The chef also joked the highly nutritious dish was so good Diana requested it "everyday" and that he would double the portion so that the palace kitchen staff could enjoy it too.

He presented the dish in a glass tumbler garnished with juicy blueberries and crunchy walnuts, explaining: "It's so much cheaper than buying it ready made at the store."

Nutrition experts have since praised overnight oats for lowering risks of diabetes and obesity and reducing cholesterol.

Celebrities such as Cameron Diaz, Gwyneth Paltrow and Victoria Beckham have publicly championed the dish in recent years for its balance of taste and health.