Sir Mark Thatcher's involvement in the alleged coup plot to topple the leader of Equatorial Guinea is understood to have begun at least a year ago.
The premier's son is understood to have reached a plea bargain
Acting on the advice of friend and alleged plot leader Simon Mann, he invested $274,000 in a company running an air ambulance in West Africa.
The boss of Air Africa Ambulance, which was to operate in Equatorial Guinea, was Crause Steyl.
Mr Steyl had been a pilot on some of Mr Mann's mercenary missions in the 1990s.
Sir Mark's lawyers maintained their client knew nothing of this aspect of Mr Steyl's career.
By the beginning of March last year, hundreds of thousands of dollars had been paid into Mr Mann's offshore bank account - including $100,000 from Mr Steyn's company, which Sir Mark had invested in.
On 6 March 2004, Mr Mann and two colleagues were arrested as they waited at Harare Airport in Zimbabwe to rendezvous with a plane carrying 64 mercenaries.
Before they could load the aircraft with weapons they were arrested in a joint operation by the Zimbabwe, South Africa and Equatorial Guinea security forces.
Mr Mann received a seven year sentence despite insisting there was no coup plot and he was trying to buy the arms for a security operation to guard a mine in DR Congo.
The other mercenaries are serving a year each in jail.
Sir Mark meanwhile is expected to plead guilty to being negligent in investing in an aircraft said to have been used by alleged coup plotters, a BBC correspondent said.