The King and Prince of Wales had time to share a joke during rehearsals for the momentous ceremony (Picture: PA)

In the 15 months since he ascended to the throne, King Charles’s unusually chunky digits have become a bizarre icon of the monarchy.

His swollen fingers have appeared in countless social media memes, been depicted in a coronation cake and were even put on a flag at Glastonbury.

And now it seem the King himself is just as happy to poke fun at them as everyone else – even in the run-up to one of the biggest days of his life.

A new BBC documentary to be broadcast on Boxing Day will show a moment of levity between Charles and his son Prince William during rehearsals for the coronation in May.

The Prince of Wales was filmed closing the delicate clasp that holds up his dad’s robes for the ceremony and joking: ‘On the day, that’s not going to go in.’

Laughing, Charles replied: ‘No, you haven’t got sausage fingers like mine.’

It’s not the first time the King has brought them up himself: in a letter to a friend after William was born, he wrote: ‘I can’t tell you how excited and proud I am.

‘He really does look surprisingly appetising and has sausage fingers just like mine.’

The documentary, named Charles III: The Coronation Year, captures the surprisingly relaxed mood in the days leading up to the historic event at Westminster Abbey on May 6.

King Charles’s fingers seem to be puffier on some days than others (Picture: Octav Ganea/Buckingham Palace via Getty Images)

Behind-the-scenes footage of the King and Queen will appear in the documentary (Picture: Leon Neal/Getty Images)

In another clip, Queen Camilla teases her husband as they go over the section of the ceremony where she is crowned.

To laughter from people nearby, she turns to Charles and says: ‘You’ve got to say yes or no.’

Archbishop of Canterbury Justin Welby joins in, telling the King: ‘If you don’t say anything, if you don’t nod or indicate, Sir, I can’t crown Her Majesty.’

The Queen then adds: ‘Don’t bother to look, I’m very happy.’

Other highlights of the film include William rehearsing the poignant moment where he kissed his father’s cheek and alterations being made to the crown jewels.

Charles III: The Coronation Year can be seen on December 26 at 6.50pm on BBC One, or afterwards on BBC iPlayer.

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King Charles is quite aware of how puffy his digits can get.