The Duke of Cambridge quenched his thirst with a sip of Irn-Bru and declared “you can taste the girders in it” during a factory visit with the Queen.

William joined his grandmother at AG Barr’s factory near Glasgow and officially opened the company’s new process facility.

Irn-Bru is popular among Scots and wider afield, and the soft drink – known for its catchy slogans – has entered folklore north of the border as the perfect hangover cure.

After touring the plant with the Queen, the duke was offered a drink and was asked by commercial director Jonathan Kemp if he had tried Irn-Bru when he was a student at the University of St Andrews in Fife.

“Not St Andrews,” he replied, but added that Irn-Bru was often a part of lunches during his time in the armed forces, and after raising his glass and sipping he said it was “delicious”.

William was intrigued when upstream manager Colin Reilly brought over a small jar containing the clear essence of Irn-Bru – with the recipe a secret only known to three people.

After taking a long sniff, the duke said: “I’m trying to guess what’s in it but that’s quite hard, isn’t it?”

Mr Reilly said: “I’d love to tell you,” and William smiled as he replied: “This is a closely guarded secret.”

Irn-Bru was launched in 1901 and has become a key brand in Scotland, rivalling whisky as the country’s national tipple.

William had tried the 1901 version of the soft drink made from the original recipe and the Queen asked about other variations on display.

The duke also seemed taken with Mr Kemp’s kilt made from a tartan featuring the orange and blue Irn-Bru colours.

The Queen is carrying out a series of events over the next four days with members of her family as part of her traditional trip to Scotland known as Holyrood Week.

Robin Barr, past chairman of AG Barr and great-grandson of the firm’s founder, Robert Barr, took the Queen around the plant in Cumbernauld, and when the royal party first arrived gave them a brief history of the company.

He told the royal visitors: “The most important date I can throw at you is 1901, when the family developed their own secret recipe for Barr’s Irn-Bru. With considerable commercial marketing over the years, that became known as ‘Scotland’s other national drink’, compared to the strong one.”

Many staff were introduced to the Queen and duke, from long-serving employees to younger members, and William also chatted to others who are involved in the company’s initiative to support the mental health of workers.

When William said he had drunk Irn-Bru in the Army rather than during his university days, Mr Barr joked: “He should have been consuming it at St Andrews. The Kirkcaldy depot which serves St Andrews opened in the 1920s – we’ve been serving that area for a long, long time.”

Only Mr Barr, his daughter, who is the company secretary, and a third unnamed person know the secret Irn-Bru recipe, and he said with a chuckle: “I can’t tell you anything about it.

“The essence was developed in 1901 really by my great-grandfather and his son, Andrew G Barr, with 32 different ingredients.”

The Queen, who has continued with her duties as head of state despite grieving for her late husband, the Duke of Edinburgh, is staying at the Palace of Holyroodhouse in Edinburgh, her official residence in Scotland.

She will be joined by her daughter, the Princess Royal, on Wednesday and Thursday.