Three private security guards who restrained the Angolan deportee Jimmy Mubenga have been cleared of manslaughter by a jury at the Old Bailey.

The 46-year-old died after being restrained by the G4S guards on a British Airways flight on 12 October 2010. Terrence Hughes, 53, Colin Kaler, 52 and Stuart Tribelnig, 39, were accused of manslaughter by forcing Mubenga’s head down and restricting his breathing as the flight prepared to take off at Heathrow airport. The jury cleared them of the charges on Tuesday after a six-week trial.

The court had heard how fellow passengers said they heard Mubenga cry out: “I can’t breathe” as he was pinned down in his seat, despite already being handcuffed from behind with his seatbelt on.

The guards said in court that they had not heard him say he could not breathe and had not pushed his head down and forward towards his knees in a position known to risk asphyxia. They said they had been restraining him to stop him hurting himself or other passengers on the plane.

The guards, who were tearful as they left the dock, said they were delighted with the jury’s decision.

In a statement issued on their behalf, Alex Preston of Olliers Solicitors said: “Terrence Hughes, Colin Kaler and Stuart Tribelnig are delighted to have been found not guilty so quickly. They bitterly regret the death of Mr Mubenga but have always said they were trying to do a very difficult job in difficult circumstances to the best of their ability. They are grateful to the judge and jury for the care they have taken resolving these sad events.”

Mubenga came to the UK with his wife, Adrienne Makenda Kambana, in 1996, but after being jailed in 2006 for actual bodily harm following a nightclub brawl he faced extradition to his home country, which he fought until 2010.

Mubenga’s widow, who sat in court 16 and at times appeared to be overcome with emotion, did not react as the jury returned the verdicts and she left the courtroom soon afterwards.

The prosecutor, Mark Dennis QC, had told the Old Bailey that Mubenga was “fit and healthy” and cooperative before boarding the extradition flight but had become upset after talking on his mobile phone in the toilet cubicle. The court also heard he became aggressive.

The guards were alleged to have responded by handcuffing him behind his back, forcing him into a seat and pinning him down in a forward-leaning position that affected his ability to breathe.

Dennis said: “Each officer would have known from their training and from common sense that keeping someone in such a position was likely to cause a person harm yet they did so over a prolonged period and did so ignoring shouts from Mr Mubenga that he was in trouble.

“‘I can’t breathe’ shouts were heard by many a passenger seated further away.”

By the time they realised Mubenga was in a critical state it was too late and he had gone into cardiac arrest, Dennis said.

Some of the 159 travellers on board recalled hearing Mubenga shouting repeatedly: “I can’t breathe.”

Nicholas Herbig, from New Mexico in the United States, told jurors: “He was saying, ‘All you people are watching them kill me. I can’t breathe. They are going to kill me.’”

In an unprecedented move, a section of the Boeing 777 with three rows of three seats was specially constructed inside the courtroom to demonstrate how Mubenga died.

Jurors were even invited to wear the rigid double lock handcuffs the guards used to experience for themselves how Mubenga would have felt.

The circumstances surrounding his death were initially made public in a Guardian investigation shortly afterwards. A subsequent investigation into forced removals exposed widespread concerns among guards about the restraining techniques used during deportations. There have also been repeated allegations of mistreatment and assaults of detainees.

The Home Office faced immediate calls to reform the system of forced deportations. Amnesty International UK said the verdict was “extremely disappointing given the multiple failings which led to the death of Jimmy Mubenga”.

Oliver Sprague, the human rights group’s security expert said: “The retelling of the grim details of how he died leaves many questions unanswered about the conduct of private security guards involved in enforced removals.

“Amnesty and others have documented numerous cases of private security companies’ wholly inappropriate conduct over the last few years, including the use of dangerous restraint techniques. We simply don’t know which of these are still being used today or if the UK government has actually delivered on its promise to introduce new and safer methods and training. The Home Office must radically overhaul the current system by placing proper controls on private security companies.”

Migrants’ Rights Network said in a statement: “This finding will do little to ease ongoing concerns about the treatment of migrants within immigration detention and during deportations from the UK. There have been widespread allegations of excessive force used against migrants during immigration removals over the past decade – a problem reportedly linked to the state outsourcing of immigration detention to private security companies.”

Jerome Phelps, the group’s detention action director, said: “Once again a migrant has lost their life in detention, and once again no one will ultimately be held to account. There needs to be a fundamental review of the detention and deportation system, to ensure that the UK does not needlessly jeopardise the lives of people who have come here in search of a better future.”

A Home Office spokesman described Mubenga’s death as “a tragedy for those involved” and said “our thoughts are with Jimmy Mubenga’s family”.

“We expect the highest standards of professionalism and conduct from our staff and contractors. Our policy has always been that restraint during removals should only be used as a last resort. Our new bespoke training package for aircraft removals, approved in June this year, will better equip our staff with practical tools to minimise the need for restraint and ensure that only the most appropriate techniques are used.”

Deborah Coles, co-director of the charirty INQUEST, said: “There needs to be a mechanism for state institutions and the private companies they employ to be held to account when people die. The lack of state accountability over black deaths in custody is a global issue and one that will not go away until urgently addressed.”