Prince Charles's ‘cruel’ six words to Diana were 'most hurtful thing he could have said'

Despite a rocky start to royal life, Sarah Ferguson was quickly welcomed into the Firm.

As a "proper country girl", had a lot in common with her new in-laws including a passion for animals, skiing and horseriding.

Her apparent ease with her new life was a complete contrast to her sister-in-law Princess Diana, who had obviously struggled to adapt.

And it wasn't long before people started to compare the two women, and a cutting comment from Prince Charles highlighted how bad things had become.

Speaking on Channel 5's Fergie Vs Diana: Royal Wives at War, royal expert Jennie Bond said: "She fitted in really well. Fergie really threw herself into the family life.

"Often Diana would be seen to be the moody one, and Fergie was the good fun, frolicking girl who was a breath of fresh air in the family."

Aware of the differences between his wife and his brother Prince Andrew's other half, Charles reportedly one day asked Diana why she wasn't more like Fergie.

Psychologist Jo Hemmings told the documentary: "There was a point at which Charles said to Diana, 'why can't you be more Fergie?'.

"For someone whose self-esteem is already pretty low, who is feeling very vulnerable.

"That wasn't just a rebute.

"The would have been one of the most cutting, hurtful remarks that her husband could have made to her."

Before tensions between the two women started to build, they were extremely close friends and it was actually Diana who set Fergie up with Andrew.

But long before that, Diana tried to get Sarah into the fold by asking the royals if she could be her lady-in-waiting - a high honour that sits somewhere between close friend and personal assistant.

However the royals rejected Diana's request, saying Sarah wasn't suitable.

Sarah's friend Lizzie Cundy said: "Sarah was told that she was actually unsuitable to be Diana's lady-in-waiting, which was a huge blow to Sarah.

"I think the Royal Family felt Sarah wasn't posh enough.

"Diana was actually quite upset about this but maybe she didn't fight for Sarah enough. "Maybe she could have put her foot down more."

Royal biographer Tom Quinn claims that the Palace were "nervous" of Fergie, who was loud and outspoken.

He told the programme: "It's not a servant and it's not a friend. It's sort of somewhere halfway between. It's a genteel occupation and gives enormous status.

"Diana was persuaded that it was too risky having Sarah Ferguson, she was seen as a loose cannon.

"She was fun, but she made them nervous."

Diana also defended her sister-in-law to other members of the royal family during her early days in the Firm.

Speaking on the documentary, Richard Kay, editor-at-large at the Daily Mail spoke one particular instance which took place not long after Sarah joined the family.

He said: "Diana had quite a set to with Princess Anne, who was complaining about something that Fergie had done.

"Diana stood up for her and said her piece. It was rather brave of Diana. She more or less asked Anne, 'you've got to give her a break. Fergie is good for the family but she's finding it hard to adjust, just as I did'."