Prince William and Kate Middleton subtly broke royal protocol during Trooping the Colour appearance

In less than 24 hours' time, the annual Trooping the Colour celebration will commence, rallying senior royals on Buckingham Palace's balcony once again.

The special occasion has prompted some spectators to wonder, however, whether Prince William and Kate Middleton might get caught out for breaking some major royal protocol, as they did during their first joint appearance in 2011.

That June, 15 years ago, the young couple had only just tied the knot. They wed two months earlier, on 29 April, in a grand Westminster Abbey ceremony, having dated since leaving university in 2004.

William and Kate's joint appearance on the balcony for their first married Trooping the Colour would, apparently, serve as a test.

Sure, they'd been papped out and about together on numerous occasions since they took their relationship public, and their royal wedding had been one of the most-watched television events in history, with an estimated two billion global viewers and over 72 million live streams on YouTube.

William and Kate tied the knot in April 2011 (Max Mumby/Indigo/Getty Images)

But they hadn't yet made any formal royal appearances since exchanging vows.

By most accounts, they fulfilled expectations by standing politely behind the late Queen Elizabeth, then Prince Charles and Camilla Parker-Bowles, and a number of other senior royals.

According to one body language expert, however, one blink-or-you'll-miss-it moment has since gone down in history as William and Kate's first breaking of traditional protocol on PDAs.

"During earlier Trooping the Colour ceremonies, it wasn’t the done thing for royals to show affection," Darren Stanton told us, on behalf of Betfair Casino. "But when it comes to William and Kate, they’re quite affectionate."

He went on to recall: "We saw William put his arm around Kate while on the balcony."

At one point during the 2011 celebration, William placed an arm around Kate (Mark Cuthbert/UK Press via Getty Images)

Apparently, however, William's smooth moves failed to phase Kate, who Stanton claims came across her usual confident self, despite only having been a working royal for a matter of weeks.

"Kate appeared completely confident at her first Trooping the Colour," he gushed. "Kate has always been fairly confident; we could see she was taking everything in her stride.

"We get genuine smiles. She’s quite animated, and her body language is very open and authentic."

Trooping the Colour describes an annual military parade held in central London to celebrate the official birthday of the British Sovereign.

Falling every June, it's also a day designated as King Charles III's Birthday Parade, with the spectacular display of pageantry being performed by personal troops from the Household Division on Horse Guards Parade.

The celebration is also designated as the King's official birthday (Rasid Necati Aslim/Anadolu via Getty Images)

It's expected the ruling monarch will be joined by his wife, Queen Camilla, his eldest son and heir, Prince William, daughter-in-law Kate Middleton, and grandchildren Prince George, Princess Charlotte, and Princess Louis.