Simon Mann (second right) with others accused of plotting

Ngave Jarukemo Muharukua, 35, died in hospital in Harare on Tuesday, a week after being taken ill inside the notorious Chikurubi prison.

The cause of death was not yet known, South African officials said.

Mr Muharukua was part of a group arrested at Harare airport in March and accused of plotting to overthrow the government of Equatorial Guinea.

Two of the men were acquitted, but 67 members of the group, including Mr Muharukua, were sentenced to between one and seven years for violating Zimbabwe's immigration, firearm and aviation laws.

They claimed that they were on their way to guard mining operations in Congo, but Zimbabwean officials accused them of being en route to depose the government of Tedoro Obiang Nhuema, President of Equatorial Guinea.

Investigation

The official Zimbabwe state news agency reported that Mr Muharukua had died, but did not reveal the cause of death.

Reports in South Africa speculated that he may have contracted a form of meningitis.

South African foreign affairs spokesman Ronnie Mamoepa told the AFP news agency: "The minister of foreign affairs has been informed of the death of a South African in Harare.

"He was transferred from Chikurubi prison to a local hospital where he died yesterday (Tuesday).

"We are in the process of investigating the circumstances leading to his death."

Former British army officer and veteran African security contractor Simon Mann was convicted of masterminding the alleged coup and handed a seven-year prison term.