Roger Hallam, the co-founder of JSO and Extinction Rebellion, was handed a five year sentence -- thought to be the longest such sentence handed in the UK for a non-violent protest.

The others, Daniel Shaw, 38, Louise Lancaster, 58, Lucia Whittaker De Abreu, 35, and Cressida Gethin, 22, were sentenced to four years imprisonment each.

The activists were found guilty of conspiracy intentionally to cause a public nuisance last week after meeting on a Zoom call and agreeing to cause disruption to traffic by climbing onto the gantries over the M25 motorway.

The protests took place across four days in November 2022 with dozens climbing gantries over the motorway which encircles Greater London and is one of the country's busiest.

Sentencing them at Southwark Crown Court in south London, Judge Christopher Hehir said: "The plain fact is that each of you some time ago has crossed the line from concerned campaigner to fanatic.

"You have appointed yourselves as sole arbiters of what should be done about climate change."

The climate campaign group, which wants the phasing out of all oil and gas use, said the sentences were "an obscene perversion of justice" given for "nothing more than attending a Zoom call".

- 'Appalled' -

At the start of the trial, Michel Forst, the UN Special Rapporteur for Environmental Defenders, issued a statement in response to complaints about the "persecution, penalisation and harassment" of Shaw.

In it, Forst warned that sentencing Shaw to more than two or more years could "violate" the UK's commitments under international law.

During the trial, Just Stop Oil, which has carried out a number of high-profile protests, claimed that the judge had ruled that climate issues were "irrelevant and inadmissible".

The group quoted David King, who was the government's chief scientific adviser between 2000 and 2007, as saying the sentences were "disgraceful".

The UN previously criticised the "severe" sentences handed to climate protesters, after two JSO activists were jailed for two and three years after scaling the Queen Elizabeth II Bridge over the River Thames at Dartford, east of London.

In a letter to the government, UN special rapporteur for climate change Ian Fry warned the sentences could stifle protest and were "significantly more severe than previous sentences imposed for this type of offending in the past".