Sir Mark Thatcher has been ordered by a South African court to face questions set by Equatorial Guinea investigators over a failed coup in the African state.
Sir Mark has denied he bankrolled the plot
The subpoena will see the son of the former British prime minister Margaret Thatcher questioned under oath in a Cape Town court on Friday.
Sir Mark, 51, denies any involvement in the plot, which was thwarted in March.
He was arrested in August at his home in South Africa and released after a bail payment of £167,000 ($314,000) was posted.
The businessman had sought to have the subpoena set aside, saying the questions would infringe on his right to silence and would impact on his trial in South Africa as well as Equatorial Guinea should he later be extradited.
Judge Deon van Zyl rejected this argument, saying South African Justice Minister Brigitte Mabandla was within her rights to seek Thatcher's responses to the Equatorial Guinea questions.
"She was perfectly entitled to accept the existing scenario as an issue of foreign policy and co-operative relations between South Africa and Equatorial Guinea," Judge van Zyl said, adding that Sir Mark's rights would not be compromised.
"At no stage have such rights been violated or even
threatened," he said.
Sir Mark was already due to appear in a South African regional court on Thursday accused of financing a bid to oust Equatorial Guinea leader Teodoro Obiang Nguema Mbasogo, charges which he denies.
The west African country, the continent's third biggest oil producer, says it intends to extradite him to face similar charges as 14 other suspected foreign mercenaries currently on trial there.