Animal rights activists have targeted the first official portrait of King Charles since he became monarch, pasting a large image of a well-known animated character over his face, in a protest about welfare on farms.

Footage from campaign group Animal Rising shows two of its supporters approaching the portrait in London’s Philip Mould gallery, and overlaying the king’s face with a big cartoon image of Wallace from the Wallace and Gromit animated comedy franchise.

They also pasted a large speech bubble, saying: “No cheese, Gromit. Look at all this cruelty on RSPCA farms!”

The aim of the supporters’ actions, Animal Rising said, was to highlight cruelty on farms granted “Assured” status, a guarantee of higher welfare standards, by the RSPCA animal charity of which Charles is the patron.

The RSPCA wasn’t impressed.

“We are shocked by this vandalism of His Majesty King, our Patron’s, portrait. We welcome scrutiny of our work, but we cannot condone illegal activity of any kind,” an RSPCA spokesperson said, adding the charity took any allegations about its Assured certified farms seriously.

The attack is the latest by activists on artwork in Britain. Climate protesters in May attempted to smash the case holding an original Magna Carta text in the British Library, and threw soup over Vincent van Gogh’s painting Sunflowers at London’s National Gallery in October.

“With King Charles being such a big fan of Wallace and Gromit, we couldn’t think of a better way to draw his attention to the horrific scenes on RSPCA Assured farms,” said Daniel Juniper, one of the two activists involved, in a statement on the group’s website.

He hoped Charles would “seriously reconsider if he wants to be associated with the awful suffering across farms being endorsed by the RSPCA”.

Buckingham Palace said it had no comment. Animal Rising said its posters would be easily removable without causing damage to the painting.

The portrait of Charles by artist Jonathan Yeo received a mixed reception after its unveiling in May, with some criticising its vibrant red colour.