Five environmental activists who organised protests that brought part of the M25 to a standstill over four days have been jailed.

Forty-five Just Stop Oil protesters climbed gantries on the motorway in November 2022, forcing police to stop the traffic, in an attempt to cause gridlock across southern England.

Judge Christopher Hehir said Roger Hallam, 58, Daniel Shaw, 38, Louise Lancaster, 58, Lucia Whittaker De Abreu, 35, and Cressida Gethin, 22, had “crossed the line from concerned campaigner to fanatic”.

At Southwark Crown Court, Hallam was sentenced to five years’ imprisonment while the other defendants each received four-year jail terms.

The sentences are the longest since the introduction by the last government of the new law of conspiracy to cause a public nuisance, in a bid to clamp down on disruptive protests.

The court heard the intention was to block most of the M25, preventing traffic from other roads from joining the motorway.

The action resulted in chaos on the M25 over four successive days, resulting in nearly 51,000 hours of driver delays, the court heard. The protests closed parts of the motorway in Kent, Surrey, Essex and Hertfordshire.

People missed flights, medical appointments and exams. Two lorries collided, and a police motorcyclist came off his bike during one of the protests on 9 November 2022 while trying to bring traffic to a halt in a “rolling road block”.

Prosecutors alleged the protests led to an economic cost of at least £765,000, while the cost to the Metropolitan Police was put at more than £1.1m.

A Zoom call chaired by Shaw was infiltrated and recorded by a Sun newspaper journalist and passed to the police, the court heard.

Judge Hehir said the recording revealed the “intricate planning and the level of sophistication involved” in the protest action.

He said the defendants were “parading their political views” by appointing themselves as “sole arbiters of what should be done about climate change”.

Hallam, a veteran environmental campaigner, was described as the “ideas man” of the movement, while the judge said Shaw was “up to his neck” in the planning of the protest.

Whittaker de Abreu and Gethin were arrested close to the M25 dressed to climb the gantries that cross the motorway.

Lancaster rented a safe house for activists due to take part, and bought climbing equipment.

Representing herself in court, Lancaster said the impact of climate change meant it was a “perilous and critical point in human history” and added that “all other means of democratic persuasion have failed”.

Gethin, from Dorstone in Herefordshire, told the judge: “It was always my intention to limit the harm caused by the disruption.” But, she said, it would not have happened if “those in power had been taking their responsibilities seriously”.

A barrister for Hallam said the 58-year-old had since rejected direct action campaigning due to its limitations and changed his approach to more conventional political campaigning.

The judge responded that the campaigner had “turned the trial into a piece of direct action protest.”

During the proceedings, Hallam was arrested three times for disobeying the orders of the judge.

He also encouraged 11 supporters to go to the court with signs saying: “Juries have a right to hear the whole truth.”

Some arrived with placards stating: “Jurors have an absolute right to acquit a defendant according to their conscience.”

As a result the judge, apparently concerned that this could affect the jury’s decisions, ordered the arrests of 11 protesters for contempt of court.

However, Judge Hehir confirmed on Thursday he had dropped the charges, saying Hallam had “orchestrated” the protest at court, making those who responded to his call less culpable.

Hallam said in a statement during the trial: “The corruption of our judges by the carbon state has crossed a line in the sand.

“This is an opportunity, and an obligation, to act. We only have a limited amount of time to halt the unimaginable horrors of climate and social collapse – and to save our democracy.”

The law of conspiracy to cause a public nuisance, which was introduced in 2022, outlaws direct action that causes “serious harm” to a section of the public. This can include property damage, injury, serious distress, annoyance or inconvenience.

In April 2023, Morgan Trowland, who scaled the Queen Elizabeth II bridge, was jailed for three years for an offence under the new legislation.

The judge in the M25 case argued that Parliament had made clear it saw non-violent direct action against national infrastructure as serious and passed a law allowing him to hand down sentences of up to 10 years – more than for some violent offences.