Following a nine-week trial, a jury at Chelmsford Crown Court today found 15 anti-deportation activists guilty of endangerment at an aerodrome, under legislation brought in to combat international terrorism. Sentencing has been reserved until 4 February 2019.

Over 18 months earlier, on the evening of 28 March 2017, “the Stansted 15”, as they have become known, cut a hole in the perimeter fence at a remote part of Stansted Airport. They then walked just over 100m to a parked Boeing 767 operated by Titan Airways, which was being prepared for a deportation flight to Nigeria.

Four of the activists locked themselves together around the Boeing’s nosewheel. The others swiftly erected a tripod behind the portside wing, and chained themselves around it. The police arrived within eight minutes, arresting one activist immediately, but it took ten hours for a specialist police dispersal team to release the others. The airport authorities closed the runway for around 80 minutes, diverting 23 flights. The Titan Airways flight was cancelled.

This is not the first time that activists have used these sorts of tactics to disrupt flights at a major airport. In July 2015, activists occupied the runway at Heathrow. In September 2016, activists occupied the runway at London City airport. Ten years ago this week, 49 Plane Stupid activists cut through the perimeter fence at Stansted, causing the runway to be closed for over three hours and 52 flights to be delayed. On that occasion – as with the later actions at Heathrow and London City – the defendants were charged with and convicted of aggravated trespass, and given non-custodial sentences.

But not this time. On this occasion, the Crown Prosecution Service upgraded the charges to endangering the safe operation of an aerodrome, or of persons at an aerodrome, “by means of devices and a substance”. This obscure charge is set out in section 1(2)(b) of the Aviation and Maritime Security Act (AMSA) 1990, introduced in the wake of the Lockerbie bombing, and a translation of the 1988 Montreal Protocol on international terrorism into British law.

It is unclear why the CPS chose this particular offence, given the aims of the protesters. Offences themselves are rarely tailored to fit specific instances, and are framed in general terms. As a result, what matters is if the terms of the offence are covered in the specific actions of the defendants. Any political purposes will be assessed, if at all, in the application of defences.

Any prosecution under AMSA requires the express sign-off of the Attorney General – in this case, Geoffrey Cox. This measure is a safeguard: it is designed to protect defendants from excessive and inconsistent charging. Cox, however, acceded to the Director of Public Prosecutions’ request to bring the charge. The defence applied for his reasons to be disclosed, but this application was rejected, so we do not know on what basis the AG thought this not to be a disproportionate application of the law.

The maximum penalty for the Stansted 15 is life imprisonment. This reflects the origin of this act as legislation on terrorism. Since its introduction, it has only been brought once before, for a case of a pilot who intentionally flew his helicopter at the control tower at Coventry airport, a month after the 11 September 2001 attacks, although his motive was not political.

In contrast, the demonstration of “endangerment” in this case has involved the prosecution poring over minute details, arguing that moving a tripod a few metres whilst erecting it means the action was “botched”, thus uncontrolled, thus inherently risky. The offence that the protestors were charged with required that the prosecution prove that their actions “endangered” the airport, and so much of the case concerned whether the defendants’ actions were dangerous. What constituted the precise risks to the airport (or persons thereat) was never clarified during the case – though the prosecution emphasised the fear felt by its main witness, an experienced deportation flight pilot, at the sight of 15 activists in pink high-vis jackets approaching the plane with banners and scaffolding poles.

The trial judge, HJH Morgan – who found with the prosecution on all significant points of legal argument – ruled that the phrase “by means of devices and a substance” did not necessarily denote the use of explosives or flammable liquids, and could include lock-ons, tripods, and protective foam.

Between us, we observed every day of the trial. Far from deliberating on serious armed attacks at airports, it has often felt over the last three months that the jury was being asked to consider breaches of health and safety regulations, with Judge Morgan emphasising in his summing up to the jury that being airside whilst unauthorised could inherently be seen to be risky.

It is important, therefore, to look at what differentiates this case from the previous protests mentioned above. Because this protest was not like the others: it expressly challenged the detention and deportation practices of the Home Office.

Throughout the trial, the seven defendants who gave evidence repeatedly spoke of their ethics of care and concern for the safety of specific named people on the flight, and for people being deported more generally – because of the conditions in which the UK government enforces the hostile environment it has created, the practices of the private companies to whom it has outsourced deportation, and the potential fate faced by deported people in the territories they are flown to. As a result of this protest, eleven of the 57 people due to be deported on the Titan flight that night currently remain in the UK. The defence pleaded the action to be lawful, justified by the prevention of a greater harm; Judge Morgan ruled this defence inadmissible.

More widely, key prosecution arguments may also ring alarm bells. Though Judge Morgan ruled that the charge did not necessarily have to concern a case of terrorism – and could concern simple acts of disruption – the prospect of a terrorist attack was very much present in the prosecution’s reasoning. By holding a protest at an airport, the prosecution argued, the protesters were responsible for what we could term secondary, or indirect, endangerment; by diverting police attention and resources, the airport would be vulnerable were a terrorist attack to have been staged at the same time elsewhere at Stansted that night.

By implication, therefore, protesters are responsible not just for their own actions, but for those unknown actions of unknown people, however speculative. Moreover, the principle would apply to all protests held at airports, whether airside or landside. By extending AMSA to acts of peaceful protest, therefore, the DPP and AG are in effect using the spectre of terrorism to turn airports into spaces of democratic suspension.

The jury evidently agreed. This verdict is a particularly cold blast in the increasingly chill wind blowing against public dissent in the UK. Most prominently, it directly places severe penalties on any future protest against deportation flights, providing the prosecution could demonstrate that any kind of object or substance was used in the protest, in whatever way. But any protests at airports could fall into this category, whether over deportation, climate change or, say, zero hours contracts.

This chilling effect is of a piece with wider trends in the policing and charging of non-violent protest. Along with more aggressive policing and charging of anti-fracking protests, the use of widespread injunctions against non-specific groups, and the huge scale of police undercover work against non-violent protest groups, the use of anti-terrorism legislation in this case is further evidence of a climate of repression targeting groups that carry out non-violent but disruptive protests. For Amnesty International, who also had observers in court throughout the trial, “The rights and freedoms of all of us are being eroded. The UK should not be targeting human rights defenders in this way.” As Ben Smoke, one of the Stansted 15, said ‘The fact that we were found guilty of this has huge ramifications on the ability of people to engage in what is a very long tradition of direct action in this country. It is part of the contract of our democracy.”

It is this, and its potential consequences for freedom of assembly and expression in the UK, that makes the Stansted 15 trial perhaps the most important political trial of our times.

Steven Cammiss is senior lecturer in law at the University of Leicester. Brian Doherty is professor of political sociology at Keele University. Graeme Hayes is reader inpolitical sociology at Aston University.

This article was updated on 11 December 2018 to clarify that Geoffrey Cox does not have a title.