Animal rights protesters have targeted a portrait of the King, pasting over the monarch’s face with the head of the animated character Wallace from Wallace and Gromit.

Demonstrators from the Animal Rising group used a window fill and water to plaster the comic posters – an image of Wallace’s head and a speech bubble – on top of the red-hued painting of Charles by Jonathan Yeo.

Daniel Juniper, 29, and Ben Thomas, 22, said members of the public “chuckled” and “loved it” when they staged the stunt at the Philip Mould Gallery in London just after midday on Tuesday.

No damage was done to the painting – the first official one of the King to be completed since his coronation – thanks to the protective layers of Perspex already in place in case of such attacks.

Animal Rising said it organised the protest to highlight alleged cruelty on RSPCA Assured farms.

Mr Juniper said: “We put up the posters nice and smoothly and as soon as we did people started chuckling behind us.

“There was a buzz around the gallery, people came and talked to us, took photos of us and then we were politely asked to leave.

“People came up and said they loved it.”

The speech bubble read in capitals: “No cheese, Gromit. Look at all this cruelty on RSPCA farms!”

The group said its “comic redecoration” was a “light-hearted action” which “played on the King’s love of Wallace and Gromit”.

The Queen once revealed that inventor Wallace and his dog Gromit – the stop-motion animation stars of hit Aardman films including The Wrong Trousers and A Grand Day Out – were her husband’s “favourite people in the world”.

The King is royal patron of the RSPCA, and Animal Rising called on the monarch to suspend his support for the charity following a “damning investigation” into more than 40 RSPCA Assured farms published last weekend.

Mr Juniper added: “We wanted to get King Charles’s attention in a way that he would enjoy and therefore be more likely to listen to us.”

Gallery owner Philip Mould said the unusual additions to the portrait were only up for 10-15 seconds before being taken down by staff, with the portrait’s clear cover wiped and dried within a minute.

“We had anticipated that there might be these types of responses. The painting is safely secured in its frame with protective layers,” he said.

“One always lives with that thought these days. I wasn’t hugely surprised.”

Yeo’s depiction of Charles shows the monarch bathed in a dramatic blood red hue.

Some commentators described it as like a poster for a horror movie, and others said it appeared as if the King was “burning in hell”, while Camilla praised its likeness to the King.

Mr Mould said up to 10,000 visitors had seen the royal painting at the gallery in the last two weeks, with many moved by the work.

“It’s had a mixed reception in the media. But the picture is only really fully understood when you see it in the flesh, a combination of its scale and the complexity of the brushstrokes, and I think a lot of people have been visibly moved by it,” he said.

A police report has been filed and security is being reviewed at the gallery.

An investigation, released by Animal Rising on Sunday, contains findings from investigations on 45 farms across the UK featuring chickens, pigs, salmon, and trout.

It alleges 280 legal breaches and 94 breaches of Defra (Department of Environment, Food and Rural Affairs) regulations, with Animal Rising calling on the RSPCA to drop the scheme.

An RSPCA spokesperson said the charity was “shocked by this vandalism of His Majesty (the) King, our patron’s, portrait”.

They added: “We welcome scrutiny of our work, but we cannot condone illegal activity of any kind. Our staff and volunteers work extremely hard rescuing, caring for, and speaking up for animals.

“Animal Rising’s sustained activity is distracting from our focus on the work that really matters – helping thousands of animals every day.

“We remain confident that our RSPCA Assured scheme is the best way to help farmed animals right now while campaigning to change their lives in the future.”

It said an urgent investigation had already been launched into the allegations.

Animal Rising describes itself as a non-violent, people-powered organisation working towards a sustainable future where humanity shares a positive relationship with animals and nature.

Buckingham Palace declined to comment.