Activists known as the Stansted 15 who were convicted under a terrorism-related law for chaining themselves around an immigration removal flight at Stansted airport have been ordered back to court over the same incident in a move they have described as “cruel and vindictive”.

Earlier this month, the activists received suspended sentences or community orders after they were convicted of endangering the safety of an aerodrome following a 10-week trial. The offence carries a potential life sentence.

The activists, who are appealing against those convictions, have received letters from Essex magistrates court ordering them to appear in April on an aggravated trespass case.

They had initially been charged with this offence before the terror-related charge was added.

On Monday, the activists’ lawyer, Raj Chada, of Hodge, Jones & Allen solicitors, sent an urgent letter to Essex CPS urging them to drop the aggravated trespass case.

In his letter, seen by the Guardian, he said the notice that they must return to court has caused the activists “great anxiety and distress”. He questioned whether it was in the interests of justice to pursue the case given that prosecutors have already secured convictions against all the activists in relation to the protest incident.

The 15 activists broke 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.

A spokeswoman for the protesters said: “This latest threat of prosecution is cruel and vindictive. After spending well over half a million pounds on prosecuting and convicting us of a piece of draconian terror-related legislation, to spend more money on trying us for yet another offence isn’t just wasting money, it’s playing cold-hearted games with our lives.

“This malicious prosecution is a window in on the kind of psychological punishment people seeking asylum in this country face from the Home Office every single day. Our current immigration system is vicious – that’s why we will not stop standing together to challenge it.”

Five of the UN’s special rapporteurs on human rights wrote to the UK government on 1 February urging them to “refrain from applying security and terrorism-related legislation to prosecute peaceful protestors”. They have given the government 60 days to respond.

Amnesty International observed the Stansted 15 trial, recognised the 15 who were tried and convicted as human rights defenders and wrote to the attorney general expressing their concern over the use of terror-related legislation in this case. It described the conviction as a “crushing blow” for human rights in the UK.

The CPS has been approached for comment.