Kate Middleton and Prince William
Kate Middleton was not happy about Prince William’s amorous advances to another woman. Pictured: Middleton and Prince William take part in an art and craft session during their visit to Evelina London Children's Hospital on Dec. 11, 2018 in London. Getty Images/Yui Mok

In the past, Kate Middleton was not happy with Prince William’s “amorous advances” to another woman.

In the early stages of the Duke and Duchess of Cambridge’s relationship, they had a “trial separation.” Prince William was the one who asked for it because he needed space. However, his behavior during that time didn’t really make Middleton happy.

Katie Nicholl revealed in her 2010 book “William and Harry” that Prince William’s attention was caught by “stunning heiress” Isabella Anstruther-Gough-Calthorpe amid his breakup with Middleton. This woman caused some tension between Prince William and Middleton.

Isabella was the sister of Prince William and Prince Harry’s polo-playing friend, Jacobi. She’s also the daughter of banking heiress Lady Mary-Gaye Curzon.

According to Nicholl, while Middleton was the “girl-next-door pretty,” Isabella “boasted cover-girl looks, a title, and a stately pile to boot.”

Prince William was really into the exquisite-looking socialite. Unfortunately for the second-in-line to the throne, Isabella wasn’t interested in joining the royal fold.

“Sadly for William, she had no aspirations to date a prince and despite his amorous advances declared that she was not interested,” Nicholl wrote.

The royal expert added that after a few months, Prince William and Middleton got back together. However, the Duchess of Cambridge gave Prince William a condition.

"Word had reached her of William’s visits to Isabella and she insisted that William was not to contact her again,” Nicholl disclosed.

Despite the issues, Isabella was among Prince William and Middleton’s guests during their royal wedding in 2011. She eventually got married to actor Sam Branson two years later.

Meanwhile, Middleton was also “deeply hurt” when Prince William visited heiress Anna Sloan during their trial separation. Anna lost her dad in a tragic shooting accident and they bonded over the loss of their parents. However, Middleton suspected that Prince William had feelings for Anna.

Aside from Isabella and Anna, there was another pretty woman who also caught Prince William’s attention before he married Middleton. In 2003, Prince William and Middleton celebrated their 21st birthday separately. Both were present at each other’s birthday bash.

However, Prince William snubbed Middleton during his birthday party because his crush Jecca Craig was there. Middleton’s glances at Prince William during his birthday party were “beyond platonic” but when she showed up at Prince William’s celebration, she barely registered to him.