Three guards who worked for the security firm G4S have been told they will not face manslaughter charges over the death of Jimmy Mubenga, an Angolan refugee who collapsed while being escorted on a flight from Heathrow airport in London 21 months ago.

The men worked as guards for the firm, which was contracted to escort deportees for the Home Office when the incident occurred. G4S has come under intense criticism in recent days after admitting it has failed to supply enough guards for the Olympics.

Mubenga, 46, died after losing consciousness on British Airways flight 77 to Angola, as it waited to take off on the runway.

The Crown Prosecution Service said that in order to prosecute the guards for "gross negligence manslaughter" they would need to prove he was held in a "severely splinted position" – bent over with his head either on or below his knees and his diaphragm restricted – for a sufficient period of time to show that the actions of the security guards were more than a minimal cause of his death.

Gaon Hart, senior crown advocate in the CPS special crime division, said there were "conflicting witness accounts" about the manner of the Angolan's restraint, although counsel found there was a "breach of duty" in the way Mubenga was held.

"In light of this, the experts unanimously concluded that given Mr Mubenga's physiological condition, having been in an agitated state before he died, they could not rule out that his death may have been caused by a combination of factors such as adrenalin, muscle exhaustion or isometric exercise," he said.

"It is not enough to tell a jury what may have caused a person's death; I have to have sufficient evidence that there is a realistic prospect of proving it to them beyond reasonable doubt, and in these circumstances the evidence did not satisfy that test."

Hart said that while there were shortcomings in the training G4S provided its guards, there was insufficient evidence to bring a charge of corporate manslaughter against the company.

"I have considered whether there is sufficient evidence to prosecute three security guards, or their employer, G4S Care and Justice Services UK Limited (G4S) for the tragic death of Jimmy Mubenga," he said.

"After very careful consideration of all the evidence, and consultation with medical experts and experienced counsel, I have concluded that there is insufficient evidence to bring any charges for Mr Mubenga's death."

Makenda Adrienne Kambana, Jimmy Mubenga's wife, said the family was "distraught". "He was crying for help before he was killed. We can't understand why the officers and G4S are not answerable to the law as we or any other member of the public would be," she said.

The Home Office initially said Mubenga had been "taken ill" while on the flight on 12 October 2010. G4S used similar wording, saying Mubenga "became unwell", forcing the plane to return to Heathrow. Passengers were taken off the flight, which was postponed for 24 hours, and interviewed by detectives from Heathrow CID.

However, after an investigation by the Guardian, which tracked down witnesses on the aircraft who said Mubenga had been forcibly detained by the guards, and was complaining he could not breathe, the Metropolitan police's homicide unit took over the case and arrested the three guards. They were later interviewed under caution for suspected manslaughter.

For months, Met detectives have been looking for passengers who were on the flight, and have sought expert medical opinion about the possible causes of Mubenga's death. The case has been with the CPS since January. There is now likely to be an inquest.

Mubenga had been in the UK for 16 years after fleeing Angola with his wife, Makenda Kambana, in 1994. His wife arrived with the couple's first son in March 1994 and Mubenga joined them three months later. After a protracted legal battle he was granted exceptional leave to remain. Over the following 16 years the couple moved across London and had another four children. Mubenga worked as a forklift driver and was a devoted family man, his widow said, and the couple eventually settled in Ilford, Essex, with their children, now aged 16 to seven months.

In 2006, Mubenga was convicted of actual bodily harm and sentenced to two years in jail following a brawl in a nightclub. After serving his sentence he was transferred to an immigration detention centre and his lawyer said he has been in and out of detention ever since as the family fought to stop him being deported.

Four G4S whistleblowers who submitted evidence to the home affairs select committee in the aftermath of Mubenga's death said the company had been warned repeatedly by its own staff that potentially lethal force was being used.

Hart said that in light of the perceived shortcomings in G4S training, the director of public prosecutions, Keir Starmer, had asked him to write to the UK Border Agency, the National Offender Management Service and the security firm to highlight the concerns raised by the experts in this case.

The co-director of Inquest, a charity that helps the families of people who die in custody, said the decision was extremely disappointing.

"Yet again, there is a failure of the state to prosecute following the use of force," Deborah Coles said.

"This is a shameful decision that flies in the face of the evidence about the dangerous use of force used against people being forcibly removed and the knowledge base that existed within G4S and the Home Office about the dangers of restraint techniques. It once again raises concerns about the quality of the investigations into deaths following the use of force by state agents and the decision-making process of the CPS."

She called for a "far reaching, effective and prompt inquest, with the full involvement of his family".

Mark Scott of Bhatt Murphy the family's solicitor said, "The family are devastated that the circumstances of Mr Mubenga's death and the people restraining him will not be called to explain their actions in criminal proceedings. The DPP's decision not to prosecute is deeply troubling. The evidence is that Mr Mubenga died after crying for help whilst under restraint."

G4S said in a statement on Tuesday that the care and welfare of those in its custody had "always been its top priority".

"A death in custody is both tragic and unacceptable. It will be for the UK Border Agency, working with the current contractor to assess whether a review of the existing guidelines on control and restraint may be appropriate."