Prince Charles' mean Fergie comparison the 'most hurtful thing he could have said' to Princess Diana

Diana Princess of Wales and Prince Charles in 1985. Photo / Getty

Despite a rocky start to royal life, Sarah Ferguson quickly became a favourite of the royal family.

She was a "proper country girl", with a passion for animals, skiing and horseriding, all

activities and passions her royal in-laws enjoyed.

Fitting in well,

Ferguson took to the royals like a duck to water.

In stark contrast, her sister-in-law Princess Diana was struggling to adapt to life in the spotlight as a royal.

Friends, family and media started comparing the two women - and pressure soon got to Prince Charles.

Speaking on Channel 5's Fergie Vs Diana: Royal Wives at War, royal expert Jennie Bond said:

"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."

With Prince Charles feeling the heat, he made a six-word comment to Princess Diana which, to this day, has been described as the "most hurtful thing he could have said".

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 rebuke.

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

Diana and Ferguson were good friends before tension grew - Diana actually set up Fergie with Andrew.

Fergie got on with the royal family very well due to a host of shared interests. Photo / Getty

Diana also tried to get Ferguson into the royal fold by asking if she could be her lady-in-waiting.

But her request was rejected by the royals, saying Ferguson wasn't suitable.

Royal biographer Tom Quinn claims that the Palace was "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."

Diana reportedly took on Princess Anne, who had complained about something Ferguson had done.

Diana effectively asked Anne to cut Ferguson some slack, saying "Fergie is good for the family but she's finding it hard to adjust, just as I did."