Kate Middleton has been trying to play peacemaker for her husband Prince William and his younger brother Prince Harry, but her efforts have been unsuccessful, a report has claimed.

The Duchess of Cambridge had forged a close bond with her brother-in-law, who often attended engagements with Middleton and Prince William as a royal trio after she married into the royal family. 

An unnamed source told Us Weekly that Middleton has not given up trying to save Prince William and Prince Harry's relationship, which reportedly continues to be strained, two years after the Duke of Sussex and his wife Meghan Markle stepped back from their roles as senior working royals and moved to California.

"There's a small glimpse of hope of saving the brothers from never speaking again: Kate," the insider claimed. "She can sense that despite everything that has happened, William still feels the loss of Harry."

The brothers have seen each other since Prince Harry's royal departure, but there's still tension between them, according to the outlet. During the Platinum Jubilee celebrations earlier this month, the Cambridges and the Sussexes were not seen publicly interacting when all four attended the National Service of Thanksgiving at St. Paul's Cathedral in London on June 3.

Middleton is "frantically trying to play peacemaker," the tipster claimed, but her efforts have been unsuccessful so far as both Prince William and Prince Harry are "stubborn."

According to the insider, the duchess talked to them separately and even called Prince Harry in his home in Montecito, California, suggesting that he reach out to Prince William on his birthday. The Duke of Cambridge celebrated his milestone 40th birthday Tuesday.

"The issue is William and Harry are both overly stubborn, so there's not much hope," the source added.

Several royal experts have spoken about Middleton's efforts in trying to reconnect Prince William and Prince Harry in recent years.

Last year, Nick Bullen, editor-in-chief of True Royalty TV, told Us Weekly that "it's pretty clear that the Duchess of Cambridge was doing her best to build the bridges. Softly, softly seems to be the order of the day."

Royal expert Katie Nicholl, who has written several books on the royal family, previously told Entertainment Tonight: "It's always been my understanding that behind the scenes, Kate has tried very hard to forge a rapprochement and act as a peacemaker between William and Harry."

Prince Harry and Markle extended an olive branch to Prince William and Middleton while they were in the U.K. for the Platinum Jubilee earlier this month, Page Six reported.

The Duke and Duchess of Sussex reportedly invited them to their daughter Lilibet's first birthday party at their U.K. home Frogmore Cottage, unnamed sources told the outlet. However, Prince William and Middleton, who attended at least two Jubilee events that day, reportedly declined the invitation.

"Things are still fraught — William is still wary of spending any time alone with Harry as you never quite know what may be reported back afterward," one U.K.-based insider told Page Six.

Prince William, Kate Middleton and Prince Harry Prince William, Kate Middleton, and Prince Harry during The Patron's Lunch celebrations for The Queen's 90th birthday at The Mall on June 12, 2016, in London. Photo: Getty Images/Jeff Spicer