In July, five of the 16 activists who brought the appeal were stunned after being sentenced to between four and five years in prison for planning in an online call to block the M25 motorway around London, a key transport link for the capital.

Roger Hallam, 58, a co-founder of Just Stop Oil and Extinction Rebellion, saw his original five-year sentence imposed in the conspiracy case cut to four years.

Two other co-accused had their sentences shortened from four years to three, with two others seeing their four-year terms reduced to 30 months.

In the January appeal, defence lawyers had called the sentences imposed against all 16 activists involved in four separate protests "manifestly excessive".

The 16 activists also included some who threw tomato soup on Vincent van Gogh's "Sunflowers" painting at London's National Gallery. Their sentences of between 20 months and two years were maintained in Friday's ruling.

Lawyer Danny Friedman said in January that the collective prison terms of between 15 months to five years were "the highest of their kind in modern British history".

The activists "did what they did out of sacrifice" and were acting in the "best interests of the public, the planet and future generations," he said, asking for the jail terms to be reduced or quashed.

But prosecutors argued the sentences had been merited as "all of these applicants went so far beyond what was reasonable".

Their actions also presented an "extreme danger" to the public and to themselves, they said.

Lady Chief Justice Sue Carr handed down the ruling on Friday, in a case which NGOs and activists have warned could have far-reaching implications for future protests.

The hearing is being closely watched amid fears that peaceful protest risks being stifled in Britain.

Activist groups Just Stop Oil (JSO) and Extinction Rebellion have launched high profile protests in recent years to fight the use of fossil fuels, which scientists say are causing global warming and climate change.

- Stonehenge, Darwin's tomb -

Just Stop Oil, which is urging the government to ban fossil fuel use by 2030, is known for its eye-catching stunts at museums, sports events and shows, but has attracted criticism over its methods.

Environmental NGOs Greenpeace and Friends of the Earth had backed what they called "a critically important legal appeal over the right to protest".

Other cases are still before the courts, including charges brought against two Just Stop Oil members accused of throwing orange paint powder over the stone megaliths of Stonehenge, as well as two activists charged with spray-painting the tomb of naturalist Charles Darwin in Westminister Abbey.

"The plain fact is that each of you some time ago has crossed the line from concerned campaigner to fanatic," judge Christopher Hehir said, during their original sentencing.

The country's previous Conservative government took a hostile stance towards disruptive direct action, and passed laws toughening punishments for such offences.