The estranged brothers were just seven miles apart as they paid their respects to the nation's longest-reigning monarch.
But the pair are not expected to meet while Harry carries out his first visit to the UK in five months, with their long-running rift showing no sign of abating.
Harry flew in from the US for a four-day run of charity visits, but began his solo trip with a poignant pilgrimage to his grandmother's grave in St George's Chapel, Windsor Castle.
The duke, who was later pictured attending the WellChild Awards in London, laid a wreath and flowers privately at Elizabeth II's tomb in the King George VI Memorial Chapel on Monday morning.
Later, at the WellChild Awards, he joked about "challenging" sibling relationships with one of the award winners.
"Does he drive you mad?" asked the duke, and Declan replied "no, we get on fine" - and Harry quipped with a smile "you know what - siblings".
Meanwhile, William and the Princess of Wales were a short car ride away in Sunningdale, Berkshire, as they visited the National Federation of Women's Institutes (WI) in tribute to the late Queen's decades-long association with the organisation.
The prince was in an upbeat mood as he and Kate joined WI members for a slice of cake, chatting fondly about his late grandmother's passion for horses.
"I think she would have liked a few more winners," William said, asking for members' memories of the late Queen and telling them "can't believe it's three years - goes quite quick".
Harry claimed in his controversial memoir, Spare, that William had physically attacked him in a row over Meghan, and that his brother and Kate encouraged him to wear a Nazi uniform to a fancy dress party in 2005 and "howled" with laughter when they saw it.
The duke - who levelled other accusations at William, Kate and the King and Queen Camilla in his Oprah interview, Netflix documentary and his autobiography - told the BBC in May that Charles will not speak to him because of his court battle over his security, and he does not know "how much longer my father has".
Harry is reported to have not been offered a place at a royal palace and is said to be staying at a hotel at his own expense.
It is not known whether the duke will see his father the King, with whom he has a troubled relationship, while in the UK.
The King, who is currently at Balmoral in Aberdeenshire, and Camilla have no public engagements planned for this week.
Harry and Charles last met face to face more than a year and a half ago when the duke made a transatlantic dash to see the monarch in February 2024 after hearing of his cancer diagnosis, but they spent just over 30 minutes together before the King left to recuperate in Sandringham.
Harry has previously spoken of his hopes for a "reconciliation" with his family, saying: "Of course, some members of my family will never forgive me for writing a book. Of course, they will never forgive me for lots of things."
He added: "But you know, I would love reconciliation with my family," and said there was "no point in continuing to fight anymore".
Senior aides to the King and the duke were pictured together in London this July in what was reported to be an initial step towards opening channels of communication between the two sides.
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