Prince Harry and his wife Meghan joined Prince William and his wife Kate on a walkabout among crowds near Windsor Castle on Saturday, after the death of their grandmother Queen Elizabeth II, raising the prospect of a rapprochement between the brothers.

The two sons of King Charles III, once so close after the death of their mother Diana in a Paris car crash in 1997, have fallen out in recent years after Harry and Meghan gave up their royal titles to move to the United States.

The couple had been in Britain on a brief visit when the queen, Britain's longest reigning monarch, died on Thursday.

Like other senior members of the royal family, Harry rushed to be by the queen's side as she deteriorated at Balmoral Castle in Scotland, and a spokesperson for the heir to the throne said an invitation for the Windsor walk had been extended by William.

A royal source described it as an important show of unity at an incredibly difficult time for the family. They emerged from the same car, all wearing black.

The two couples did not interact much during the 40-minute walkabout near one of the queen's favorite English homes, as they stopped to read messages left among the flowers and shake hands and chat with the thousands of people lining the Long Walk.

At one point Meghan was hugged by a woman in the crowd while others stretched to shake hands and talk to the four royals in the early evening. William spent some time bending down chatting to children, while Kate and Harry accepted flowers and messages of encouragement from those in the crowd.

Royal observers had been looking for signs of a detente amid the pomp and ceremony of a period of national mourning and Elizabeth's state funeral.

On Friday Charles used his first address to the nation as king to express his "love for Harry and Meghan as they continue to build their lives overseas."

He also bestowed on William and daughter-in-law Kate the titles of Prince and Princess of Wales, which he and his late wife Diana previously held.

William was heard telling one well-wisher that the days following the queen's death had been "so surreal."

"We all thought she was invincible," he said.

Under royal rules, the monarch's grandchildren are automatically eligible to become princes or princesses of the realm, so now that Harry's father Charles is king, his children, Archie, 3, and Lilibet, 1, can receive those titles. Lilibet was named after the queen's childhood nickname.

It is not clear whether they will take the titles.

"At the moment we're focused on the next 10 days. As and when we get information, we will update the (royal) website," a spokesperson for Charles told reporters.