Prince William enjoys Irn-Bru during Queen's factory visit

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 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 wad 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 Colin Reilly, upstream manager, 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.