Pathologists carried out tests for 220 drugs and poisons on Chief Abiola's body.

UN Secretary-General Kofi Annan: Met Abiola shortly before his death

Dr James Young, the Canadian leader of the team, said they had found narrowed arteries and an enlarged heart - signs of long-term heart disease of the kind which can cause sudden death.

Chief Abiola, Nigeria's most prominent political prisoner, died while meeting US diplomats and as speculation mounted that he was about to be released.

His supporters and family said they believe he was either killed or that his treatment while in prison contributed to his death.

An initial post mortem indicated death by natural causes but the pathologists from Canada, the US and Britain, spent a month carrying out more detailed tests.

No drugs or poisons

Dr Young, Ontario's chief coroner said: "At this time our final opinion is that death was due to natural causes as a result of his long-standing heart disease.

"Toxicology tests indicated the presence of anti-malaria and anti-inflammatory drugs, all of which were expected to be found, given Chief Abiola's medical history. No other drugs or poisons were detected."

Chief Abiola votes in 1993

Dr Young said his team was aware of the concerns of Chief Abiola's family that his treatment in prison was a contributory factor in his death.

He said: "We continue to believe that these concerns are understandable and deserve further consideration.

"Further comment on these issues is beyond the scope of the investigation we were asked to undertake."

Chief Abiola, 60, was the presumed winner of elections in 1993 which were annulled by the military. He was jailed in 1994 for declaring himself president.

Widespread rioting followed his death on July 7.

A month earlier General Sani Abacha, Nigeria's ruler, died and was succeeded by Major-General Abdusalam Abubakar.

Within days of taking power he began releasing some of Nigeria's political prisoners.