A former inspector of prisons has described the decision not to prosecute three G4S guards over the death of Angolan national Jimmy Mubenga as "perverse."
Lord Ramsbotham said he could not understand why the Crown Prosecution Service had written to the firm expressing concerns but then failed to take legal action.
Mr Mubenga collapsed and later died after he was forcibly restrained on a plane at Heathrow Airport in October 2010 during an attempted deportation.
Witnesses reported that Mr Mubenga had cried out that he couldn't breathe shortly before he collapsed.
Three G4S Terence Hughes, Stuart Tribelnig and Colin Kaler were interviewed under caution on suspicion of manslaughter.
But on Tuesday, Senior Crown advocate for the CPS Gaon Hart said that there was insufficient evidence to bring charges against the three civilian guards.
To bring a prosecution for gross negligence manslaughter it would need to be proved that the guards' actions were "more than a minimal" cause of his death, he said.
The decision was roundly condemned by asylum campaigners and human rights groups.
Lord Ramsbotham, who is conducting an independent investigation into the removal of detainees for Citizens UK, said an experienced coroner should now carry out an investigation into the death as soon as possible.
An investigation found there was a "breach of duty" in the way Mr Mubenga was restrained and that the guards may not have received adequate training in the use of the positional asphyxia restraint hold, the peer told the House of Lords on Wednesday.
There was also evidence they failed to spot warning signs that Mr Mubenga was suffocating.
He said: "I understand that the Crown Prosecution Service is writing to the UK Border Agency,National Offender Management Service and to G4S expressing (their) concerns.
"I have to say that in the face of all the evidence gathered during our inquiry, quite apart from all the other evidence which was available, I find CPS's decision, at kindest, perverse.
"Passengers reported hearing Mr Mubenga cry out that he couldn't breathe and that the guards were killing him. There had been warnings from the Home Office to G4S in 2006 about the dangers of using positional asphyxia."
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