Prince William and Duchess of Cambridge splashed with water by London Marathon runner

While the king-in-waiting seemed amused, the Duchess appeared less than impressed

The Duke and Duchess of Cambridge might be experts in remaining regal at all hours of the day but every now and again even they can’t help but let the mask slip.

One such occasion was witnessed at the London Marathon on Sunday. Prince William and the Duchess were handing out drinks to thirsty runners near the finish line, only for one of them to unexpectedly return the compliment by hurling water in their direction.

The aquatic assault caught the Prince directly in the face, with a few droplets hitting the immaculately-turned-out Duchess. The king-in-waiting seemed amused, but the Duchess appeared less than impressed, fixing the assailant with an icy glare.

A runner squirts water towards Prince William, Duke of Cambridge as he hands out water to runners during the 2017 Virgin Money London Marathon (Getty Images)

Nevertheless, it's worth noting that the majority of runners bombarded the royal couple with high fives, hugs and selfies. Prince Harry was also at the race but was not present when the water was thrown.

The three royals, whose mental health campaign Heads Together is the official chosen charity for the marathon, stood at the start line to blast the klaxon that marked the start of the race and continued to express their support for the runners at various points.

“The Duke and Duchess of Cambridge and Prince Harry are spearheading the Heads Together campaign to end stigma around mental health,” reads the charity’s website. “Heads Together aims to change the national conversation on mental health and wellbeing, and is a partnership with inspiring charities with decades of experience in tackling stigma, raising awareness, and providing vital help for people with mental health challenges.”

Record figures of runners took part in this year’s marathon. More than 40,000 athletes embarked on the gruelling 26.2 mile run.