REVEALED: The chilling piece of advice Grace Kelly gave to Princess Diana

Princess Diana and Grace, Princess of Monaco were without a doubt two of the most iconic royals of all time.

In 1956, Grace - who was formerly known as Oscar-winning actress Grace Kelly - married into the Monaco royal family when she tied the knot with Ranier III, Prince of Monaco.

Fast forward twenty-five years to 1981, a young Lady Diana Spencer married into the British royal family, exchanging vows with the future king, Prince Charles.

The two Princesses met two months before Diana's royal wedding when the then 19-year-old attended her first official royal engagement alongside the Charles.

According to royal biographer Andrew Morton, during the glamorous gala at the Royal Opera House in London, Grace shared some chilling advice with the "people's princess".

Writing in his book, William & Catherine: Their Lives, Their Wedding, Morton said: "She chose a black ballgown with plunging, gravity-defang décolletage, which had commentators wondering if she had exposed more than she should, for a charity gala in the presence of Princess Grace of Monaco, the former Hollywood star Grace Kelly.

Morton revealed the Princess of Wales later told him: "It was a horrendous occasion.

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"I didn’t know whether to go out of the door first. I didn’t know whether your handbag should be in your left hand not your right.

"I was terrified, really – at the time everything was all over the place.

According to Morton, the Princess of Monaco, who was 51 at the time, could apparently see how uncomfortable Diana was.

"Sensing her discomfort, Princess Grace took her aside and listened as she poured out her heart," the biographer claims.

"'Don’t worry,' Princess Grace had joked, 'it will get a lot worse.'"

The following year, in 1982, Diana welcomed her first child with Charles – Prince William. It was also the year Grace Kelly tragically passed away as she was driving back from her country home to Monaco with her daughter Stephanie.

Fifteen years later, in August 1997, Diana also died after being involved in a tragic car accident in Paris.