The Queen is to celebrate her 96th birthday at her Sandringham estate in Norfolk, staying in the cottage where the Duke of Edinburgh lived much of his final years.
The monarch was expected to fly by helicopter on Wednesday from Windsor Castle to Sandringham and stay until early next week, after an eventful Easter break that saw the Duke and Duchess of Sussex visit her.
The Queen is believed to be staying at Wood Farm, the "loved" cottage that Prince Phillip made his home following his retirement from public life.
Her birthday celebrations on Thursday are expected to be typically low-key with friends and family expecting to visit in the coming days.
In recent months the monarch has been experiencing mobility issues and has cancelled appearances at a number of high-profile events, but she did attend Philip's memorial service and has continued carrying out her duties of state and hosting virtual events.
Harry said his grandmother was "on great form" when they met before he and wife Meghan travelled to Holland for the Invictus Games.
But he pointedly added he wanted to make sure she was "protected" and had "the right people around her", but did not elaborate in the NBC's Today show interview whether he was referring to palace aides or the royal family.
The Queen has previously spoken about Wood Farm, when she hosted a rare public event at Sandringham on 5 February, the eve of her Platinum Jubilee.
Teresa Thompson, 70, the retired housekeeper of Wood Farm, said after chatting to the Queen that day: "All the royal family love Wood Farm, because it's out of the way, it's small, It's intimate. You haven't got lots of officials and household, it's just the close staff.
"And they literally can relax in the family house. It's wonderful, and I had a wonderful 22 years down there. It was the best time of my life without a doubt."