Four South African mercenaries and a UK citizen granted pardons for a coup plot in Equatorial Guinea will not be prosecuted, the National Prosecutions Authority (NPA) said on Thursday.

"The Acting National Director of Public Prosecutions, advocate Mokotedi Joseph Mpshe SC, has decided not to prosecute UK citizen Simon Mann and the four South African citizens," NPA spokesperson Mthunzi Mhaga said in a statement.

Mann and South Africans Nick du Toit, George Alerson, Sergio Cardoso and Jose Sundays were convicted in a trial which implicated Mark Thatcher, the son of former British Prime Minister Margaret Thatcher, as the financier of a 2004 plot to overthrow the oil-rich Equatorial Guinea and oust long-serving President Teodoro Obiang Nguema.

Their release coincided with a visit by President Jacob Zuma.

Mhaga said certain acts relevant to the conspiracy involving the five were performed in South Africa.

"In addition, certain other persons resident in South Africa had in one way or the other been involved in the conspiracy."

Mhaga said according to South Africa's law regulating foreign military assistance, it was a serious offence for any South African resident to perform any action aimed at overthrowing a foreign government.

"The acts performed by the five pardoned persons clearly fall within the ambit of the above piece of legislation."

However, Mhaga said that according to the constitution and other legislation, a person already acquitted or tried for a crime cannot be tried again.

"The constitution is therefore in favour of the five persons and constitutes a bar to any such prosecution."

Mhaga said the NPA considered all contraventions of the Regulation of Foreign Military Assistance Act in an "extremely serious light".

"The convictions which have been handed down to date have been as a result of international cooperation."