Mann, who was sentenced in July 2008, admitted conspiring to oust President Teodoro Obiang Nguema during his trial.
The former special forces officer apologised, saying he was not the most senior coup plotter.
The Foreign Office said it was aware of reports of Mann’s proposed release and was seeking to clarify the situation.
A spokesman said: "This is something for the Equatorial Guinea authorities at this stage."
A statement on the Equatorial Guinea’s Ministry of Information website said Mann had received a full pardon.
Mann was held in 2004 with 64 others in Zimbabwe before being extradited.
His extradition came after he had served four years in prison in Zimbabwe for trying to purchase weapons without a licence.
Equatorial Guinea, an oil-rich former Spanish colony, has been ruled by President Obiang since he seized power from his uncle in 1979.
Mann’s lawyer had asked for leniency, saying his client was a pawn of powerful international businessmen and he had been "not a co-author" of the coup plot but "an accomplice".
Mann had implicated Sir Mark Thatcher, son of UK former Prime Minister Margaret Thatcher, and London-based millionaire Eli Calil as organisers of the plot.
Sir Mark, who now lives in southern Spain, was fined and received a suspended sentence in South Africa in 2005 for unknowingly helping to finance the plot.
After Mr Mann’s verdict, he reiterated to the BBC that he had no direct involvement. He said he had known nothing about any plan to overthrow the government and added that he had already faced justice in South Africa.