Five South African policemen accused of killing the anti-apartheid activist Steve Biko in 1977, giving rise to world-wide protests, will not be prosecuted, the government said on Tuesday.

Despite his inquest being told that Biko had been driven naked, shackled and unconscious in the back of a police van from Port Elizabeth, where he was arrested, to a police cell in Pretoria, where he died of brain injuries, the South African justice ministry found that there was insufficient evidence to support a murder charge, partly because there were no eyewitnesses to the killing.

Charges of culpable homicide and assault were also considered, but because the black consciousness movement leader died just over 26 years ago, it was no longer possible to prosecute.

"If at a later stage new evidence emerged that cleared the problems we've had, we would reconsider our decision. With a matter like this, we can't leave it hanging, we must have finality," said Chris MacAdam, a lawyer for the National Prosecuting Authority.

In 1999, the Truth and Reconciliation Commission, formed to help heal apartheid's emotional and psychological wounds, denied amnesty to the five men. To qualify, applicants had to prove a political motive for their crimes and tell the commission the full truth of their activities.

All five policemen said Biko tried to attack one of his interrogators while in custody in Port Elizabeth, and claimed his head accidentally hit a wall as they tried to restrain him. Although barely conscious, he was left chained to a metal gate in a standing position for two days before being taken to Pretoria.

The killing of the 30-year-old activist caused world outrage, especially after Jimmy Kruger, the justice minister at the time, said Biko's death "leaves me cold".

He became a martyr in South Africa's townships, which had been in turmoil since the Soweto uprising of 1976, and an international symbol of resistance to apartheid, after his story was told in the Richard Attenborough film, "Cry Freedom" and Peter Gabriel recorded a tribute to him which topped the charts.