Fifteen activists convicted of a terrorism-related offence for chaining themselves around an immigration removal flight at Stansted airport have received suspended sentences or community orders.

The judge decided not to imprison them after he accepted they were motivated by “genuine reasons”.

Amid an outcry over what human rights defenders branded a heavy-handed prosecution, the group, who have become known as the Stansted 15, were convicted last December of endangering the safety of an aerodrome.

They had broken into Stansted airport’s “airside” area in March 2017 and chained themselves together around a Boeing 767 chartered by the Home Office to deport 60 people to Nigeria, Ghana and Sierra Leone. After a 10-week trial a jury found them guilty of the charge – an offence that carries a potential life sentence.

At Chelmsford crown court on Wednesday, Judge Christopher Morgan QC, dismissed submissions in mitigation that the group should receive conditional discharges for the direct action protest, which briefly paralysed the airport, saying they did not reflect the danger that had been presented by their actions.

He said such action would “ordinarily result in custodial sentences”, but that they “didn’t have a grievous intent as some may do who commit this type of crime”. The mood in the court had lightened considerably at the start of the hearing when Morgan said that he did not consider the culpability of any of the defendants passed the threshold of an immediate custodial sentence.

The heaviest sentences were reserved for three of the group who had been previously convicted of aggravated trespass at Heathrow airport in 2016.

Alistair Tamlit and Edward Thacker were sentenced on Wednesday to nine months in jail suspended for 18 months, along with 250 hours of unpaid work. Melanie Strickland was sentenced to nine months suspended for 18 months, with 100 hours of unpaid work.

Benjamin Smoke, Helen Brewer, Lyndsay Burtonshaw, Nathan Clack, Laura Clayson, Mel Evans, Joseph McGahan, Jyotsna Ram, Nicholas Sigsworth, Emma Hughes and Ruth Potts were each given 12-month community orders with 100 hours of unpaid work, while May McKeith received a 12-month community order with 20 days of rehabilitation.

In mitigation, Dexter Dias QC said it should be taken into account that all acted to try to help individuals they perceived to be in danger. “The reason they wanted to prevent [the flight’s] departure is that they believed the welfare and safety of some of the people on that flight was at risk,” he said.

“In those circumstances the court historically in this country have considered that conscientious motivations offer quite significant mitigation.”

Dias pointed out that 11 of those who had been due to be deported to west Africa that night remain in the country, including two of whom there were reasons to believe were victims of human trafficking, and two who were subsequently found to have been victims of human trafficking. “One of them had been raped and forced into sex work in several European cities,” he said.

Kirsty Brimelow QC, who appeared to have been specially recruited for the mitigation after not acting for any defendant during the trial, told Morgan he must balance the defendants’ rights to protest and free association against the harm their actions caused the airport.

Brimelow last year acted for three fracking protesters whose sentences were overturned by the court of appeal as “manifestly excessive”. She continually referred to that case as she told Morgan that he must consider the “proportionality” of the sentences.

The defendants emerged from the court to a rousing reception from hundreds of supporters who had spent the day protesting outside. Tamlit said he was “relieved that’s over”.

“It’s been a gruelling process,” he said. “The flight that went this morning [to Jamaica] put things in perspective. We might have been in jail tonight but people could have visited us and we would have eventually been released.

“Not going to jail is a partial victory but we are going to keep campaigning to end charter flights, immigration detention and the hostile environment.”

McKeith’s mother, Ag, said she was pleased at the relatively lenient sentence. But, she said she felt they ought not to have been convicted at all. “Despite the judge’s stern account, it’s simply not true that they endangered anybody at the airport,” she said. “The only people who were in danger were the people on the plane. I watched the trial all the way through and watched the prosecution trying to spin straw into gold, and they didn’t convince me.”

Graeme Hayes, reader in political sociology at Aston University, who observed the entire trial, said: “Although the defendants have not got the custodial sentence, the bringing of a terrorism-related charge against non-violent protesters is a very worrying phenomenon. It’s so far the only case [of its type] in the UK, and points to a chilling of legitimate public dissent.”

The defendants have already filed an appeal against their convictions. Raj Chada, of Hodge, Jones & Allen, represented most of them. “We will be studying the judgment carefully to review whether there are any issues that need to be brought up in the appeal,” he said.

“It’s striking that nowhere was there any endangerment of individuals identified.”