“If the flawless Kate has anything resembling an Achilles’ heel, it’s Isabella," Ms Ostler explained, referring to society girl Isabella Branson, who William was once said to be smitten with.
“Formerly Isabella Anstruther-Gough-Calthorpe, she is the leggy blonde with whom Prince William was once said to have been smitten, even supposedly turning to her during his break from Kate.”
William and Kate had a well publicised split in 2007, when the Prince was unsure of committing to his university sweetheart and pursued a party lifestyle instead for a few months.
And while Isabella is now married to Sam Branson, there are reportedly still tensions between her and the Duchess of Cambridge.
Ms Ostler continued: “While she was invited to the royal wedding, she has not become friends with Kate — which is unusual, as the Duchess makes it a habit to be friendly to any women with whom William has history, such as the brunette ex Jecca Craig or socialite Davina Duckworth-Chad.”
The details of Kate's concerns around his exes come after it came to light that Beatrice and Eugenie have felt ousted by the Duchess ever since she married Prince William.
According to resurfaced reports, the York sisters have felt pushed out by Kate and even blame Queen Elizabeth for letting the Duchess take centre stage.
Writing back in May 2016, Catherine Ostler explained in the Daily Mail that there was a “cold relationship” between Kate, Beatrice and Eugenie, as the Queen appeared to be putting Kate into the spotlight more than her own grandchildren.
The reporter added: “It particularly hurt when Kate did solo engagements with the Queen during the Diamond Jubilee.
“They felt snubbed.”