A UK judge on Friday jailed two climate activists who threw soup at Vincent van Gogh's "Sunflowers" at London's National Gallery in 2022 for terms of two years and 20 months respectively.

Just Stop Oil protesters Phoebe Plummer, 23, and Anna Holland, 22, were convicted at London's Southwark Crown Court of criminal damage in July.

Greenpeace UK's co-executive director, Will McCallum, called the sentence "a draconian and disproportionate punishment for a protest that caused minor damage to a picture frame".

Plummer and Holland had pleaded not guilty over the incident in October 2022.

Sentencing the pair, Judge Christopher Hehir said the painting could have been "seriously damaged or even destroyed".

"Soup might have seeped through the glass. You couldn't have cared less if the painting was damaged or not," he added.

"You had no right to do what you did to 'Sunflowers'."

The gallery, located in Trafalgar Square, said the protesters caused "minor damage to the frame" but the painting itself was protected by a screen and was unharmed.

Holland, who received the 20-month term, and Plummer, who was jailed for two years, also glued themselves to the gallery wall during their protest.

- 'What matters most?' -

"What is worth more -- art or life?" Plummer had shouted.

"It's another grim milestone in the ongoing crackdown on peaceful protest waged by the last government," Greenpeace UK's McCallum said.

"Protest is by its nature inconvenient and occasionally messy. These defendants do not deserve to spend years behind bars for standing up for a liveable planet."

Just Stop Oil wants an end to the extraction and burning of fossil fuels like oil, coal and gas, because the greenhouse gas emissions they create are "driving us towards climate collapse... (that) spells disaster for human societies globally".

It has staged a number of high-profile stunts in recent years to draw attention to their plea to "end fossil fuels before they end us".

The group has targeted the Wimbledon tennis tournament and British Open golf tournament, as well as art galleries and museums and a performance of "Les Miserables".

Five activists, including the climate group's founder, were given between four and five years in jail in June for conspiring to plan protests that blocked the M25 orbital motorway around London.

Just Stop Oil says climate change poses an existential crisis for humanity and that its direct tactics are justified.

In July 2022, Just Stop Oil protesters glued themselves to John Constable's pastoral masterpiece "The Hay Wain", also in the National Gallery.