The family of a man found dead in an immigration detention centre have reacted with fury to a decision to drop criminal charges against two private firms.

Last year, the Crown Prosecution Service (CPS) decided that charges should be brought over the death of Prince Fosu, whose naked body was found on the concrete floor of his cell in Harmondsworth, west London, on 30 October 2012.

On Tuesday, the CPS announced that it was reversing its decision, saying that it did not believe it could secure convictions. The news came on the sixth anniversary of Fosu’s death.

Charges under health and safety laws were dropped against GEO Group UK Ltd and Nestor Primecare Services Ltd. GEO ran Harmondsworth at the time, while Nestor was responsible for the healthcare of those detained there. Both companies are alleged to have breached section 3 of the Health and Safety at Work Act 1974 by failing to take reasonable care of health and safety.

Fosu, 31, was from Ghana. He was booked on a flight to leave Britain on 5 November 2012. He had been in the immigration detention centre for six days before he died. He had been arrested after being found naked in the street. A check on his immigration status found that he had overstayed his visa.

A CPS spokesperson said: “We have a duty to keep all cases under continuing review. Following a review of the evidence in this case, we considered the challenges in bringing a prosecution were greater than we had previously assessed and concluded the case should not continue. Our thoughts remain with the family of Prince Kwabena Fosu.”

Fosu’s father, who did not wish to be named, said: “For a bereaved family, it is difficult to understand why there is such delay in the criminal decision-making process. We are just told that it is normal.

“How can I now explain to the wider family on the anniversary of Prince’s death that we have got nowhere in six years? How can I explain why the prosecution decision has been reversed? How can it be acceptable to treat a bereaved family this way? This is not justice.”

The campaign group Inquest said Prince, who was suffering from mental ill health, was held for six days in unsanitary conditions, without any clothing or bedding or a mattress. “It is thought that Prince went into sickle cell crisis and died on the floor of his cell,” the group said.

Deborah Coles, the director of Inquest, said: “We had hoped this trial would shine a spotlight on the inhumane treatment of Prince, who died naked on the concrete floor of his cell. On the sixth anniversary of Prince’s death, this is a reprehensible step backwards by the CPS.

“Recent months have seen the highest number of deaths of immigration detainees on record. The culture of impunity for private companies and the Home Office, in the face of well-documented ill treatment of detainees, is shameful.

Kate Maynard, a solicitor, said: “The CPS is clearly a failing and arrogant institution if they think that the management of this case has been acceptable. This case illustrates the systemic delay for decision-making in death in custody cases. To take four years to review a case, announcing that there will be a prosecution, and then reversing that decision 18 months later, is dysfunctional. A culture change is required within the CPS to fix the way it operates so that bereaved families receive a decent, efficient and humane service.”

• This article was amended on 2 November 2018. An earlier version attributed to Deborah Coles of Inquest a part of a quote which should have been attributed to Kate Maynard. This has been corrected.