The lengthy multi-year sentences handed to Just Stop Oil activists are “not acceptable in a democracy”, a UN special rapporteur has said, as the government faced growing pressure to reverse the previous administration’s “hardline anti-protest” approach.

Michel Forst, the UN special rapporteur for environmental defenders, joined a growing chorus of voices condemning the sentences handed down to the five defendants for planning non-violent protests on the M25.

But the government said it would not intervene in the case, with Keir Starmer’s spokesman saying judgments and sentencing were matters for independent judges, “and it is not for politicians to intervene”.

Daniel Shaw, Louise Lancaster, Lucia Whittaker De Abreu and Cressida Gethin were each sentenced to four years in prison this week after being found guilty of planning disruptive protests on the M25. A fifth defendant, Roger Hallam, was sentenced to five years by a judge who said he “sat at the very highest level of the conspiracy”.

Forst, whose role is to protect individuals facing penalisation, persecution, or harassment for exercising their environmental rights, attended two days of the trial earlier this month as he attempted to intervene with UK authorities on behalf of Shaw. He called the jail terms “punitive and repressive”.

“We are really shocked by what’s happening in the UK in the case of Daniel Shaw,” Forst told the Guardian on Friday. “He has already spent more than 100 days on remand. Now, to hear that he’s been sentenced to four years in prison, that’s really shocking.

“Even if we are talking about a disruptive form of protest, and there is no denying that, it is still entirely non-violent and it should have been treated as such. For me, for my team, it’s not acceptable in a democracy like the UK.

“The second element is that it’s a very dangerous ruling, not only for environmental protesters, but also for the right to protest as such, because we understand now that those who would like to go to the street to demonstrate, to organise a rally, they would consider twice before going out.

“That’s a deterrent for the right to protest in the UK.”

He said he was in discussions to visit the UK in the autumn, and that he hoped to meet the new attorney general, Richard Hermer, to discuss the case “but more broadly what’s happening with the right to protest and democracy in the UK”.

A host of human rights campaigners joined Forst in decrying the sentences. Sam Grant, director of advocacy at Liberty, said what appeared to be a trend for increasingly severe sentences for non-violent protest indicated “a grave erosion of … freedoms” in the UK. Hanna Hindstrom, who investigates rights abuses against environmental defenders for Global Witness, said the “incredibly harsh” sentences were “a profound injustice”.

And Tom Southerden, Amnesty International UK’s human rights adviser, called on the government to repeal the portions of the Police, Crime, Sentencing and Courts Act 2022 that legislated the statutory offence of public nuisance used against the defendants.

“Today’s draconian sentences and the manner in which the trial was conducted show that the hardline anti-protest approach adopted by the previous government is being emulated by the courts,” Southerden said.

But on Friday, government officials said ministers would not intervene. Pressed on whether Labour would look again at anti-protest laws it opposed before entering government, Starmer’s spokeswoman said: “The prime minister is very clear that when it comes to these cases, the judgments and sentencing is for independent judges to make them, they’ve had all the facts and evidence before them.

“I think you’ve seen some of the reporting from those proceedings and can see some of those details for yourself too. Fundamentally, [it is] not for politicians to interfere and opine on decisions that have been taken by judges.”

The government was nevertheless coming under growing pressure to look again at the controversial laws, which critics said have led to increasingly draconian action against climate activists.

Even Doughty Street Chambers, the prime minister’s former employer, pointed out “there was no irony lost” in the fact that public nuisance, when a common law offence, was formerly often used to prosecute polluters, while “the same offence in statutory form is now being zealously deployed to prosecute environmental protesters”.

Dale Vince, the green entrepreneur, who stepped away from bankrolling Just Stop Oil to become one of the Labour party’s most significant donors, joined the broadcasters Chris Packham and Hugh Fearnley-Whittingstall in echoing calls for a meeting with Hermer about the protesters’ case.

“I think climate denial should be illegal, but instead it’s illegal to talk about the climate crisis in court,” Vince said. “Now five peaceful protesters could face years in jail as a result of this perverse ruling. It’s a travesty of justice and that’s why I’m joining the calls for the new attorney general to intervene.”

Clive Lewis, MP for Norwich South, called for action “as a matter of some urgency”, adding: “I will be backing calls for a meeting with the attorney general, I think that needs to happen with some urgency, I will be addressing this in parliament.” He called for the laws “which have allowed this to happen” to be “consigned to the dustbin of history”.

Siân Berry, the Green MP, said the sentences were “a hangover from the last government’s obsession with punishing … non-violent, peaceful protests”. They highlighted a challenge for the new government, she added, and Labour must “review the guidelines given to judges that have led to such extreme, disproportionate sentences for peaceful protest”.

On Friday, their views appeared to resonate with the public at large. An online poll conducted for Social Change Lab, a nonprofit that researches protest and social movements, found 61% of respondents agreed the sentences doled out to the five protesters were “harsh”, compared with 12% who felt the sentences were too lenient.

A spokesperson for the attorney general’s office said: “Decisions to prosecute, convict and sentence are, rightly, made independently of government by the Crown Prosecution Service, juries and judges respectively. The attorney general has no power to intervene.”