They had been prosecuted following a protest in March 2017, where they ultimately prevented a charter flight that was due to deport 60 individuals to Africa.
The group, known as the Stansted 15, were initially charged with aggravated trespass but the charge was changed to endangering safety at a public airport.
All defendants denied the offence at trial, and said they were “guilty of nothing more than intervening to prevent harm” to migrants on board the plane.
On Friday, the Lord Chief Justice, Lord Burnett, sitting with Mr Justice Jay and Ms Justice Whipple, overturned all 15 demonstrators’ convictions.
Lord Burnett said the protesters “should not have been prosecuted for the extremely serious offence ... because their conduct did not satisfy the various elements of the offence. There was, in truth, no case to answer.”
The judgment said the offence they were charged with was intended for “conduct of a different nature” after the campaigners’ lawyers told the Court of Appeal the offence used was related to terrorism and had been created in the wake of the 1988 Lockerbie bombing.
May MacKeith, a member of the Stansted 15, said almost four years of legal proceedings “should never have happened”.
“But for many people caught up in the UK immigration system the ordeal lasts much, much longer,” she added.
“The nightmare of this bogus charge, a 10 week trial and the threat of prison has dominated our lives for four years. Despite the draconian response we know our actions were justified.”
Raj Chada of Hodge Jones and Allen Solicitors, who represented the Stansted 15, said the case should be a matter of “great shame” to the Crown Prosecution Service (CPS) and attorney general.
“Both have questions to answer as to why they authorised such an unprecedented charge,” he added.
“Amnesty International adopted the 15 as human rights defenders, Liberty intervened in the case and even the UN, through their special rapporteurs, expressed concern, yet the case went forward.”
In March 2017, the defendants cut through the perimeter fence of Stansted airport in Essex and used pipes to lock themselves together around a plane.
The Boeing 767 had been chartered by the Home Office to remove 60 people to Nigeria, Ghana and Sierra Leone, and was stationary on the airport’s apron.
The trial heard the defendants believed the deportees were at risk of death, persecution and torture if they were removed from Britain, and many were asylum seekers.
Campaigners said that 11 of the 60 passengers remain in the UK, and included victims of human trafficking.
The protesters, who all pleaded not guilty, were convicted in December 2018 of the intentional disruption of services at an aerodrome under the Aviation and Maritime Security Act 1990.
A judge at Chelmsford Crown Court handed three defendants, who had previous convictions for aggravated trespass at airports, suspended prison terms and gave 12 defendants community sentences.
Judge Christopher Morgan said alleged human rights abuses, immigration policy and proportionality did not have “any relevance” to whether a criminal offence had been committed.
“In normal circumstances only a custodial sentence would have been justified in this case, but I accept that your intentions were to demonstrate.”
United Nations human rights experts raised concern over the case and warned the British government against using security-related laws against protesters and critics.
“We are concerned about the application of disproportional charges for what appears to be the exercise of the rights to peaceful and non-violent protest and freedom of expression,” a statement said in February 2019.
“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.”
The group received high-profile support from MPs and public figures, including the Bishop of Chelmsford.
An open letter signed by dozens of politicians and academics in September condemned the practice of “secret deportation flights”, which came into renewed focus following the Windrush scandal.
Amnesty International said the case was part of a Europe-wide trend of volunteers and activists being criminalised for helping migrants.
Kate Allen, Amnesty International UK’s director, said the Court of Appeal ruling was a “good day for justice”.
“The Stansted 15 will take their place in the history books as human rights defenders who bravely brought injustices perpetrated by the state into the light,” she added.
“This case should never have been brought and there must be lessons learnt for how we treat human rights defenders in this country.”
Lana Adamou, a lawyer for the Liberty human rights group, called the charges “an attack on our right to express dissent”.
“All too often it is the most marginalised in society, and those acting in solidarity with them, who bear the brunt of over-zealous policing and crackdowns on protest, making it even more important for the government to take steps to facilitate protest and ensure these voices are heard, rather than find ways to suppress them,” she added.
At November’s Court of Appeal hearing, lawyers for the activists told the court the legislation used to convict the 15 is rarely used and not intended for a protest case.
In documents before the court, the Stansted 15’s barristers argued it was intended to deal with violence of the “utmost seriousness”, such as terrorism, rather than risks of “a health and safety-type nature” posed by those who have trespassed at an airport.
Lawyers for the group also argued that the attorney general – who is required to sign off on the use of the legislation – should not have granted consent for the law to be used in this case, that the crown court judge made errors in summing up the case and in directions given to the jury.
Barristers representing the CPS had said the convictions are safe and that the trial judge was correct.
Tony Badenoch QC told the court: “We don’t accept that the act is constrained to terrorism and nothing else.”
A CPS spokesperson said: “We will consider the judgment carefully in the next 28 days.”
The 15 are: Helen Brewer, 31; Lyndsay Burtonshaw, 30; Nathan Clack, 32; Laura Clayson, 30; Melanie Evans, 37; Joseph McGahan, 37; Benjamin Smoke, 21; Jyotsna Ram, 35; Nicholas Sigsworth, 31; Melanie Strickland, 37; Alistair Tamlit, 32; Edward Thacker, 31; Emma Hughes, 40; May McKeith, 35; and Ruth Potts, 46.