Mr Hammarskjold and 15 other people died when their plane exploded just before landing in Northern Rhodesia, now Zambia. He had been trying to mediate a peace agreement between Congo and the breakaway province of Katanga.

The commission says eight letters it uncovered during its investigation into apartheid-era crimes suggest South African, British and American secret services might have been involved.

Commission chairman, Bishop Desmond Tutu, said the documents contained references to sabotage of the aircraft in which Mr Hammarskjold died.

The British Foreign Office has denied that London was involved in any plot to kill the Swedish diplomat.

A spokesman said: "Intelligence agents of the United Kingdom do not go around bumping people off. At this time during the Cold War, Soviet misinformation was quite rampant so [the letters] may have been put out by them."

BBC Correspondent Greg Barrow in South Africa says that at the time of the crash the Cold War superpowers were jostling for influence in the Congo and western governments were not entirely comfortable with Mr Hammarskjold's conciliatory approach to the demands of revolutionary African leaders.

'Hammarskjold should be removed'

The letters are said to record meetings between the South African military, the American CIA and Britain's MI5 security service.

President Tutu: uncertain if the letters are genuine

The letters are headed the South African Institute for Maritime Research (SAIMR) - thought to be a front company for the South African military.

One of the letters says: "In a meeting between MI5, special ops executive and the SAIMR, the following emerged _ it is felt that Hammarskjold should be removed."

The undated letter says: "Allen Dulles [the then Director of the CIA] has promised full co-operation from his people."

Another document gives details of orders to plant explosives in the wheel bay of an aircraft. It says they were primed to go off as the wheels were retracted on takeoff.

Documents yet to be verified

The documents were released because the commission had not had time to investigate them before its mandate expired in July, Bishop Tutu said.

"We have been unable to investigate the veracity of these documents and of allegations that South Africa or other western intelligence agencies were involved in bringing about the air crash," he added.

The documents have been turned over to President Nelson Mandela and the South African Justice Minister Dullah Omar, who will decide what further actions should be taken.

Three independent inquiries concluded many years ago that the crash was probably caused by pilot error, but did not exclude other possibilities.

Conspiracy theories about the deaths have abounded. In 1992, two former UN officials said mercenaries had shot down Mr Hammarskjold's plane.