The courts have apologised to the activists known as the Stansted 15 after they were wrongly ordered back to court following their conviction under a terrorism-related law for chaining themselves around an immigration removal flight at Stansted airport.

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.

They are appealing against those convictions and were, according to a spokewoman, ”devastated” to receive letters a few days ago from Essex magistrates court ordering them to appear on 15 April for a hearing relating to a separate aggravated trespass charge. During their sentencing for the terror-related charge the court said that the aggravated trespass charge would be adjourned while the appeal against their convictions is considered.

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

The activists were distressed and bewildered by the letter, according to their lawyer, as they believed they would not have to return to court before their appeal has been considered.

A spokesman for HM Courts and Tribunal Service (HMCTS) issued an apology to the activists on Tuesday night over the letter. He said: “This case was incorrectly listed and letters sent in error. We apologise for any inconvenience this may have caused and the hearing on 15 April will be cancelled.”

The activists’ lawyer, Raj Chada of Hodge, Jones & Allen solicitors, said the letter ordering the activists back to court has caused them “great anxiety and distress”. He questioned whether it was in the interests of justice to pursue the aggravated trespass charge given that prosecutors have already secured convictions against all the activists in relation to the protest incident.

“At least HMCTS have retracted and apologised to our clients. It doesn’t take away from the misery that our clients suffered over the weekend. More importantly, it is a temporary reprieve as the CPS are still continuing the case and want it hanging over our clients like the sword of Damocles – this is not in the public interest.”

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 Stansted 15 said receiving the letters had been “devastating for us”, adding: “This kind of legal threat and drawn-out prosecution stretching over years is a small window into the horrendous process people seeking leave to remain with their families and communities face at the hands – and the mistaken letters – of the Home Office every single day.”

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 protesters”. They have given the government 60 days to respond.