MALABO (AFP) - Equatorial Guinea on Tuesday pardoned British mercenary Simon Mann and four others jailed for a failed coup plot in the West African nation in 2004.

The country's national radio said Mann, South African partner Nick du Toit and three others had been granted amnesties. After their release they would have 24 hours to leave Equatorial Guinea.

The British and South African mercenary leaders were jailed for 34 years each for their role in a plot to oust President Teodoro Obiang Nguema as head of the former Spanish colony.

Mann, a former British Special Air Service officer, was expected to be released "imminently", British officials said.

"We understand this was a personal decision by the president of Equatorial Guinea on humanitarian grounds," said a Foreign Office spokeswoman in London.

South African President Jacob Zuma arrives in Equatorial Guinea on Tuesday for an official visit.

Mann, 57, underwent hernia surgery last year and his state of health was one of the reasons for the amnesty, national radio said, announcing a November 2 decree.

The government announcement said Mann needed "regular medical treatment near his family" and that he had shown "credible signs of repentance and the desire to be reinserted in society".

Miguel Mifuno, an advisor to Nguema, told the BBC that "Simon Mann conducted himself in exemplary fashion during his trial and his incarceration in Equatorial Guinea.

"He has had some health problems and was operated on. He is now in good health, but the president thinks he should now be allowed to live in peace with his family."

Mann's family expressed delight at the amnesty.

"The family is absolutely delighted that Simon has been pardoned and is to be released shortly," the family said in a statement.

"Everyone is profoundly grateful to the president and the government of Equatorial Guinea. The whole family is overjoyed at the prospect of finally welcoming Simon home after five-and-a-half long years away," it added.

A family spokesman said Mann was due to arrive home "in the next few days".

Mann was arrested in March 2004 along with 61 other people when their plane landed in Zimbabwe. He spent four years in a Zimbabwe prison on firearms charges before being deported to Equatorial Guinea.

At his trial there, Mann implicated Mark Thatcher, son of former prime minister Margaret Thatcher, in the plot to oust Equatorial Guinea's leader, who has ruled the country since a 1979 coup that ousted his uncle. The plot aimed to bring exiled opposition leader Severo Moto to power.

Mann, who attended the prestigious Eton school and Sandhurst military academy, was said to be the brains behind the coup attempt. He told his trial last year that Spain and South Africa had backed the plot.

Mann and Du Toit had set up Executive Outcomes, which operated from Pretoria and helped the Angolan government protect its oil installations from rebels during that country's long civil war.

Mann, who lived in the Cape Town suburb of Constantia -- also home to Earl Spencer, brother of the late Princess Diana, and until recently Mark Thatcher -- allegedly used the 'old boy' network to finance his deals, media reports said.

Thatcher pleaded guilty in a South African court to financing the planned coup and was given a four-year suspended prison sentence.

The announcement of releasing the convicted mercenaries came as Equatorial Guinea opens its presidential election campaign on Thursday. The vote is set for November 29 and Obiang Nguema is seeking another mandate.

The former Spanish colony is Africa's third biggest oil producer after Nigeria and Angola following the discovery of large offshore oil deposits in the early 1990s. The wealth has not reached the country's ordinary people.

Obiang Nguema in 1987 created the Democratic Party of Equatorial Guinea (PDGE), which dominates the political apparatus and leads a coalition of nine parties among the 13 that are legally recognised.

Since multiparty politics were introduced in 1991, the PDGE has easily won all elections.