The Duke of Cambridge gamely took a swig of Irn-Bru on Monday, declaring it "delicious" as he joined the Queen for a tour of the manufacturer’s factory in Scotland.

Prince William and his grandmother were visiting AG Barr’s factory near Glasgow, where they officially opened the company’s new process facility.

"I’m happy to try some," the Duke said as they were guided to a table set up with bottles and two glasses. "That’s very good actually. I haven’t had it for a while, but it’s very good. That’s the original one, is it?"

He told the Queen: "You can taste the girders in it."

The visit marked the start of a four-day tour of Scotland for the Queen – an annual trip known as Holyrood Week, celebrating Scottish community, innovation and history. She will be joined later in the week by her daughter, the Princess Royal.

Her Majesty, 95, who rarely eats or drinks in public, opted not to try Irn-Bru, which is particularly popular in Scotland, where it rivals whisky as the national tipple.

The Duke of Cambridge tries Irn-Bru during his visit to AG Barr's factory in Cumbernauld

Credit: Andrew Milligan/PA Wire

The Duke, who is known as the Earl of Strathearn in Scotland, 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 when he was in the Armed Forces, his Scottish colleagues drank it at lunchtime.

The Queen, shown an array of versions of the drink, asked: "Are they different?" and was told that yes, they were slightly.

The Duke was intrigued when Colin Reilly, the upstream manager, brought over a small jar containing the clear essence of Irn Bru, with secret ingredients known only to three people.

After taking a long sniff, he said: "I’m trying to guess what’s in it but that’s quite hard, isn’t it?" Mr Reilly replied: "I’d love to tell you," before the Duke smiled and added: "This is a closely-guarded secret."

Irn-Bru was launched in 1901 and has become a key brand in Scotland, where it has entered folklore north of the border as the perfect hangover cure.

Throughout this week’s tour, the Queen, who has continued with her duties as head of state despite grieving for her late husband, will stay at the Palace of Holyroodhouse in Edinburgh, her official residence in Scotland.

The Queen meets employees during a visit to AG Barr's factory in Cumbernauld

Credit: Andrew Milligan/PA

Holyrood Week, also known as Royal Week, was cancelled last year because of the pandemic. This year, in line with Government guidelines, traditional events hosting thousands of people such as the garden party at Holyroodhouse will not take place.

In a series of tweets on Sunday, Buckingham Palace emphasised the Queen’s long-standing links to Scotland.

It said: "Tomorrow, the Queen will arrive in Scotland for Royal Week 2021. Royal Week, or ‘Holyrood Week’, takes place each summer as the Queen and members of the Royal family undertake visits across Scotland celebrating Scottish culture, achievement and communities.

"Her Majesty is connected to Scotland by ancestry and deeply held affection. As well as spending family summers at Balmoral Castle, the Queen has visited almost every area of Scotland from the Outer Hebrides to Dumfries, meeting Scots from all walks of life."