King Charles' first official portrait has been vandalised by animal rights activists

In a nod to the fictional inventor, a speech bubble was also pasted on, which read: "No cheese Gromit. Look at all this cruelty on RSPCA farms."

Organisers insisted that the act was done in humour and simply wanted to highlight alleged cruelty said to be taking place on farms approved and endorsed by the RSPCA.

In a post on the group's website Daniel Juniper, one of those involved, said: "Even though we hope this is amusing to his Majesty, we also call on him to seriously reconsider if he wants to be associated with the awful suffering across farms being endorsed by the RSPCA. Charles has made it clear he is sensitive to the suffering of animals in UK farms; now is the perfect time for him to step up and call on the RSPCA to drop the assured scheme and tell the truth about animal ."

The group also posted a short video of the two activists carrying out the act.

In the past, Queen Camilla has noted how the classic characters are her husband's "favourite people in the world."

Artist Jonathan, 53, said about his sittings with the king - who is currently undergoing treatment for an undisclosed form of cancer: "It was a privilege and pleasure to have been commissioned by The Drapers' Company to paint this portrait of His Majesty The King, the first to be unveiled since his Coronation.

"When I started this project, His Majesty The King was still His Royal Highness The Prince of Wales, and much like the butterfly I've painted hovering over his shoulder, this portrait has evolved as the subject's role in our public life has transformed.

"I do my best to capture the life experiences etched into any individual."