Prince Harry once described Princess Kate as “the sister I’ve never had and always wanted”, but a royal expert has since claimed the Duke of Sus𝑠e𝑥 is now full of regret over the state of their relationship.

Kate and Harry have been estranged ever since the latter moved to the US with his wife Meghan Markle, and he wrote many negative things about her and Prince William in his memoir Spare.

Royal author Tom Quinn told The Mirror: “He really misses that warm, uncomplicated relationship, and he is torn between loyalty to his wife and regret about the loss of the woman he was so close to. Losing Kate was Harry’s second great loss after losing his mother.

“Meghan doesn’t feel guilty about Kate, because she feels that Kate should’ve been the one person in the Royal Family to back her against every difficulty.

“Apart from anything else, Meghan knew Kate was an outsider, so she just couldn’t understand why Kate always towed the royal line rather than supporting her, Meghan, the other outsider.”

Kate’s cancer diagnosis has shocked both Meghan and Harry, who sent a message of support to the Waleses after the news broke, but Mr Quinn does not believe this will be enough to patch up the feud.

Meghan and Kate have never been the best of friends, but things took a turn after a bust-up over a bridesmaid’s dress allegedly made Meghan cry, although she acknowledged that Kate later apologised.

The author added: “Kate’s cancer diagnosis has been a real shock for Meghan, but she can see no way to patch up the relationship simply because it has become so poisonous.

“She also doesn’t want people saying that she is trying to make things up with Kate just because Kate is ill. Meghan still feels Kate was in the wrong and should apologise before they try to build bridges.”

The Princess of Wales was said to be taken aback by the contents of Spare, and is not ready to forgive Harry’s portrayal of her tensions with Meghan.

The contentious row over Princess Charlotte’s bridesmaid dress took place not long after Kate had given birth to Prince Louis, with Harry claiming in his book that Kate had texted Meghan saying: “Charlotte’s dress is too big, too long, too baggy. She cried when she tried it on at home.”

The Duchess of Sus𝑠e𝑥 responded that there was a tailor on standby to alter the dresses if necessary, but Kate’s cool reply is said to have left Meghan “sobbing on the floor”.

Harry wrote in his book: “Meg asked if Kate was aware of what was going on right now. With her father. Kate said she was well aware, but the dresses. And the wedding is in four days!”

The Duke admitted that Kate “hadn’t meant any harm”, with the Princess arriving at the Sus𝑠e𝑥 home the next day with flowers and a card to make amends.

Royal biographer Duncan Larcombe told New! magazine about Harry and Kate’s relationship: “Bombshell news like Kate’s [cancer] puts things into perspective and Harry isn’t immune from being filled with regrets and what-ifs.

“Kate was like the sister Harry never had and he is probably full of regret at how his relationship with her and William has collapsed so spectacularly.”