Katie Nicholl, royal expert and author of the 2017 book "Harry: Life, Loss and Love", claims Prince Harry made his new sister-in-law Kate cry during his best man's speech in front of 300 guests at Buckingham Palace.
According to extracts quoted in the Express UK, Harry told the newlyweds their "decade-long romance was his inspiration" which prompted the Duchess of Cambridge to shed a gentle tear.
The notoriously naughty son of Princess Diana also incorporated some cheeky lines into his address, including a few humourous digs at elder brother William.
"William didn't have a romantic bone in his body before he met Kate, so I knew it was serious when [he] suddenly started cooing down the phone at [her]," the book says.
The Duke of Sussex went on to impersonate his sibling by calling Kate "Babykins" to the delight of guests.
Ms Nicholl's account claims Harry's then-girlfriend Chelsy Davy helped him to craft the speech, sensibly advising him to remove a line about Kate's "killer legs" which she felt may not have been well received by the royal entourage.
The biography outlines the close friendship Zimbabwean-born Chelsy - who dated Harry for seven years from 2004 to 2011 - enjoyed with Kate, with the royal girlfriends apparently "stuck together like glue".
Duchess Catherine's relationship with Davy stands in marked contrast to the reportedly on-going feud between her and Meghan Markle.
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