President Obiang has ruled his country for 24 years
A court on the Channel island of Guernsey has blocked the release of papers naming alleged backers of an aborted coup in Equatorial Guinea.
Lawyers representing Equatorial Guinea President Teodoro Obiang wanted the documents for a civil action against those they accuse of planning the coup.
Sir Mark Thatcher admitted unknowingly helping to finance the coup bid in a South African court earlier this year.
Briton Simon Mann is in a Zimbabwe prison for his part in the coup.
He is one of those being sued by Mr Obiang.
After legal proceedings lasting a year, the Guernsey Court of Appeal overturned a previous ruling that papers showing the details of those paying money into a set of accounts on the islands should be released.
Sir Mark, son of the former UK Prime Minister Margaret Thatcher, was given a suspended jail term and fined after agreeing a plea bargain to help South African investigators.
Another 64 suspected mercenaries travelling with Mr Mann remain in a Zimbabwe jail.
Fourteen others are in prison in Equatorial Guinea, after being found guilty of trying to topple Mr Obiang.
The leader of the advance party, Nick du Toit, was given a 34-year sentence.
The conduct of the trials in Zimbabwe and Equatorial Guinea were criticised in the West, amid allegations of torture and forced confessions.