He added: “The staff member asked us what we were doing and why and let us talk about what we wanted to talk about.
“He wasn’t negative. He was concerned for the painting and we then took the speech bubble off and they took the (face) down later.”
Staff later unpeeled the two Wallace and Gromit themed stickers and asked the protesters to leave.
‘We are shocked by this vandalism’
An RSPCA spokesman said: “We are shocked by this vandalism of His Majesty (the) King, our patron’s, portrait.
“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.”
They added that any concerns about welfare on RSPCA Assured certified farms were taken extremely seriously and an investigation had already been launched.
Wallace and Gromit one of the King’s ‘favourite films’
In 2012 the Duchess of Cornwall said her husband, the then Prince Charles, enjoyed watching Wallace and Gromit.
The Duchess revealed royal viewing habits when she met children at a reception at Clarence House.
She did so at an event where she was unveiled as patron of the Big Jubilee Lunch, which aimed to bring communities together to celebrate the Queen’s Diamond Jubilee.
She told the children: “I love Shaun the Sheep. He’s really funny. My husband Prince Charles – Wallace and Gromit are his favourite people in the world. We always sit and watch them – it’s one of his favourite films.”
The gallery has reported the incident to the police.
The Metropolitan Police was contacted for comment.
Buckingham Palace declined to comment.