You tell her, Phil.
The Duke, 91, made the gesture during a performance by American singer Alicia Keys, according to reports in the Evening Standard. Keys, who wore a black dress with a revealing overlay, was unaware of the Duke's reaction as she took to the stage accompanied by ballet dancers in the background.
She performed alongside British favourites Rod Stewart, Girls Aloud, One Direction and Britain’s Got Talent winners Pudsey and Ashleigh Butler.
And if the Duke’s verdict on the R&B singer’s performance wasn’t entirely clear, he told reporters he was “stone deaf” when asked which routine was his favourite.
“All of them [are] … but to be honest we’re both stone deaf,” he said after the show.
The ceremony at the Royal Albert hall last night marked the 100th anniversary of the event. It also preceded the Queen and Prince Philip’s 65th wedding anniversary which the royal couple celebrated privately today.
It comes as a new book is due to be published on the Duke's gaffes.
The book, written by veteran royal correspondents Phil Dampier and Ashley Walton, is the second of its kind following Duke of Hazard, which was published in 2006 to mark the duke’s 85th birthday.
One-liners included Prince Philip warning a British student that he would develop “slitty eyes” if he remained in China for too long and telling a Scottish driving instructor to “keep the natives off the booze” long enough for them to pass their tests.
Mr Dampier said: “The Duke comes across as a much more complex ad thoughtful character in this book as we’ve got a lot of his wisdom too.
It’s not just slitty eyes and pot bellies. It’s showing him in a bit more of a rounded light with more philosophical stuff – on organic farming, religion and the Queen. It’s not just one-line insults.”