Prince William and Kate Middleton declined Prince Charles' invitation for a holiday.

According to Abbie Llewelyn, a journalist for Express, the Prince of Wales invited the Duke and Duchess of Cambridge to spend quality time with each other at the Castle of Mey, the late Queen Mother's remote home in Caithness, Scotland. However, Prince William and Middleton didn't say yes.

According to the report, the reason Prince William and Middleton declined the invitation remains unknown. However, Prince Harry and Meghan Markle seemed to have taken him up on his offer as they were present.

The Duke and Duchess of Sussex were also invited, and unlike Prince William and his wife, they accepted it. The couple spent a week with the Prince of Wales, leaving the latter "joyous."

Prince Charles reportedly told friends that he was "tickled pink" when Prince Harry and Markle confirmed that they would be joining him in Scotland. During the visit, the royals were seen enjoying coastal walks and bird-spotting.

"I think it’s fair to say that Meghan has brought Harry and Charles closer, and Charles is thrilled to have a new daughter-in-law," a royal insider said.

On the other hand, Camilla Parker Bowles wasn't present at the same event. She had a "bucket-and-spade" holiday with her own children and grandchildren.

Prince Charles reportedly has a great relationship with Markle. In fact, he has a nickname for his second daughter-in-law. The future king reportedly calls the former "Suits" star "tungsten" because she is "tough and unbending."

"Prince Charles admires Meghan for her strength and the backbone she gives Harry, who needs a tungsten-type figure in his life as he can be a bit of a softy. It's become a term of endearment," a source said.

Markle is apparently as fond of her father-in-law. She reportedly even helped Prince Harry become closer to his own father.

"Meghan met Charles and was bowled over by his charm," the source said. "She told Harry he was wonderful: welcoming, warm, hard-working, kind and stable. She made it clear that he should appreciate him and bond more."

On the other hand, Prince Charles relationship with Middleton isn't as great as his connection with Markle. The heir reportedly felt that the Duke and Duchess of Cambridge are closer to Michael and Carole Middleton. In fact, there was a time when he felt that the Middleton's were the favored godparents when it comes to grandchildren Prince George and Princess Charlotte.