Equatorial Guinea issued a request for an Interpol arrest warrant for Mark Thatcher yesterday, in connection with his alleged involvement in a failed coup attempt in the west African country five years ago.

Thatcher is accused of helping the British mercenary Simon Mann to stage a coup. Interpol headquarters in Lyons said yesterday the request was being processed. Details of Thatcher's involvement have been provided by Mann, who was extradited from Zimbabwe to Equatorial Guinea two months ago.

Mann, who is awaiting trial for allegedly attempting to overthrow the government and threatening to kill the president, has already said in a Channel 4 interview from Black Beach prison in the capital, Malabo, that Lady Thatcher's 54-year-old son was "part of the team".

In 2005 Thatcher was fined £265,000 and given a four-year suspended sentence in South Africa, where he and Mann lived, after a plea bargain in which he admitted helping to finance the purchase of a helicopter for the plotters even though he suspected it might be used for mercenary activity.

The announcement of the warrant request was made on Thursday by José Olo Obono, Equatorial Guinea's attorney general. "We are not going to let this drop," he said. "We will pursue him wherever he goes." Obono was speaking to British journalists who had flown to Malabo on advice from official sources that Mann would give a press conference this week before his trial. Neither event has taken place and the trial is not expected for several weeks. In return for his cooperation Mann and his family are hoping for a reduced sentence.