A British lad was given the shock of his life after opening a 21st birthday card – which revealed he’d unknowingly been on the receiving end of a family joke for the past three years.

Sarah Austin shared the hilarious footage of her son Charlie’s big day to TikTok, which has since gone viral at his expense. The clip begins with the birthday boy opening a giant card from his parents, who asked him to “read it out”.

“Happy 21st birthday, Charlie,” he duly responded, oblivious to what is to come. “Yay – you no longer have a black box.” Grateful to his mum and dad, Charlie jumped for joy at his newfound freedom – but all was not as it seemed.

Returning to the card, Charlie continued as his jaw dropped: “I’ve had no black box for three years? Are you serious?” Laughter erupted around the garden as he voiced several expletives.

Closing the card’s message, Charlie cursed: “Are you taking the p***? You’re telling me it took six and a half hours to get to Cornwall?” His dad mockingly replied: “It slowed you down though, didn’t it?”

Charlie proceeded to ask his other guests if they were in on the gag, to which they all confirmed they had been. “We knew for years,” one friend smirked. “When you had to slow down the other day, I was crying laughing!”

The video has been viewed a staggering 1.7million times despite Sarah having little more than 500 followers. And TikTok users were quick to comment, with many praising the actions of Charlie’s parents. “Fantastic parenting! Three years safe driving in the bag,” one person declared.

A second agreed: “Three years of driving carefully and avoiding accidents. Three years of experience on the roads. Top parents right there.” A third hailed: “This is how to parent! Learning how to actually drive safely is invaluable.”

“A black box is another name for a telematics unit that’s fitted to a car to record how a car is driven,” explains Cinch. “This data determines how safe a driver you are and is then sent to your insurer, which uses it to decide on your premiums.”

“The device records a car’s speed, the distance it travels, how it accelerates and brakes, and what time of the day or night the car is used. All this information builds a profile of the driver that the insurance company uses to work out how much of a risk they are to insure. The safer a driver you are, the less you’ll have to pay for your policy. But, on the other hand, if it seems that you drive erratically or riskily, your premiums can go up.”