The groups Just Stop Oil and Extinction Rebellion have in recent years carried out spectacular, often disruptive protests, including stopping London's busy traffic and targetting famous places and paintings, as part of their campaign to highlight climate change.

But last July, five activists 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.

They were among 16 activists who appealed their jail terms before the Court of Appeal in London in January.

In the 44-page ruling on Friday, Lady Chief Justice Sue Carr cut the heaviest jail term of five years imposed on Roger Hallam, 58, a co-founder of Just Stop Oil and Extinction Rebellion, for the conspiracy.

Carr and two other judges reduced it to four years saying the original term was "manifestly excessive".

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

One of the oldest activists, Gaie Delap, 78, also saw her sentence shortened from 20 to 18 months, for scaling one of the gantries on the M25.

As Carr read the ruling, about 20 activists stood up and turned around to reveal they were wearing T-shirts emblazoned with the words "Corruption in Court" on the back.

- Tomato soup -

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

The complainants' 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

And another lawyer for the group, Raj Chada, said Friday that they might now take the case to the Supreme Court.

Prosecutors had 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.

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

And NGOs and activists have warned the case could have far-reaching implications for future protests.

"Despite some modest reductions, these sentences are still unprecedented and they still have no place in a democracy that upholds the right to protest," said Greenpeace co-executive director Areeba Hamid.

Friends of the Earth lawyer Katie de Kauwe said in a statement the group was "pleased" some of the sentences had been reduced.

She said the group welcomed that Friday's ruling and stressed that "sentencing for peaceful protest needs to factor in both the defendant's conscientious motivation, and protections afforded under the European Convention of Human Rights."

- Stonehenge, Darwin's tomb -

She added "ultimately however, we believe that locking up those motivated by their genuine concern for the climate crisis is neither right nor makes any sense".

Just Stop Oil and Extinction Rebellion launched their protests to fight the use of fossil fuels, which scientists say are causing global warming and climate change.

But the groups, which are urging the government to ban fossil fuel use by 2030, have attracted criticism over their eye-catching methods.

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 Westminster Abbey.

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