William, Kate, Harry and Meghan together at Windsor Castle
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The sight of the two couples together -- even if they separated to speak to and shake hands with different sides of the cheering crowds -- will likely spark rumours of a private reconciliation.
The two couples had last been seen together at the Commonwealth Day service at Westminster Abbey in 2020, just days before their bombshell announcement.
- Clear olive branch -
They moved to the United States, from where they have made repeated public criticisms of life in the institution, including claims of racism.
That prompted William, 40, to angrily insist that "we are very much not a racist family".
But Charles offered what has been interpreted as a clear olive branch to his self-exiled son, who now lives in California and is seeking to reinvent himself as a wellness and social justice advocate.
In his first speech as king on Friday, Charles, 73, spoke of his love for his second son and daughter-in-law.
Relations between the brothers -- from Charles' first marriage to princess Diana -- were visibly frosty when they reunited last year to unveil a statue to their mother.
They did not meet during the queen's Platinum Jubilee celebrations in June.
British media said despite their proximity the couples had no plans to meet -- until the queen's death on Thursday.
But they are now expected to stay for the queen's funeral at Westminster Abbey in London on September 19.