UN human rights experts have demanded that the UK cease the use of security and terrorism-related charges against peaceful protesters, after 15 anti-deportation activists were prosecuted for an offence carrying a potential life sentence.

They described the use of the charge – which had to be signed off by the attorney general – as disproportionate for non-violent protesters. “It appears that such charges were brought to deter others from taking similar peaceful direct action to defend human rights and in particular the protection of asylum seekers,” they said.

Stansted 15: no jail for activists convicted of terror-related offences Read more

The experts, from the UN’s Office of the High Commissioner for Human Rights, wrote to the government at the beginning of February, drawing ministers’ attention to the importance of the right to peaceful protest after the group, known as the Stansted 15, were prosecuted for endangering the safety of an aerodrome after blocking the departure of an immigration removal flight.

The rapporteurs’ letter will remain private for 60 days to give the government an opportunity to respond, a spokesman said. However, details of the letter were alluded to in a statement published on the OHCHR website.

“We express our grave concern that the 15 protesters were prosecuted and convicted under a statute which is primarily concerned with the translation of the state’s international aviation security obligations into national law – offences which are also listed under the 2006 Terrorism Act,” the UN experts said. The Crown Prosecution Service has strenuously denied that the protesters were charged with a terror offence.

“Furthermore, we believe that charging, prosecuting and convicting the 15 protesters under the Aviation and Maritime Security Act fails to take sufficient account of the statutory intent of the law, the mischief it was designed to remedy, and the potential for abuse of these statutory provisions in the circumstances of this case,” the experts added.

After a prosecution described by Amnesty International as a “crushing blow for human rights in the UK”, all 15 members of the group who staged the protest were spared jail when they were sentenced at Chelmsford crown court in Essex on Wednesday. They had been convicted after the jury in a 10-week, £1m trial were convinced that their action had put the safety of Stansted airport at risk.

On the night of 28 March 2017, they cut through the perimeter fence at the international airport, and ran to a Titan airways Boeing 767, chartered by the Home Office, which was waiting to remove 60 people from the UK to Nigeria, Ghana and Sierra Leone.

Four chained themselves around the front landing gear, while the others erected a tripod behind the left wing. One perched on top, while others locked themselves to the base to prevent it being moved.

In their defence, they had sought to argue that they acted to prevent human rights abuses, both in the UK and in deportees’ destination countries. As a result of their action, 10 of the passengers on the flight remain in the country, including four who may have or are confirmed to have been victims of human trafficking.