The former head of the Nelson Mandela Children's Fund, Jeremy Ractliffe, has resigned from the charity's board after admitting he secretly kept diamonds received from the model Naomi Campbell.
Mr Ractliffe admitted he had the gems only when Ms Campbell mentioned him at the war crimes trial of former Liberian leader Charles Taylor two weeks ago.
Prosecutors say she received the diamonds from Mr Taylor in 1997.
Mr Ractliffe had apologised for his secrecy, the charity's board said.
The former chief executive handed the diamonds over to South African police after Campbell testified that she had given three stones to Ractliffe because she wanted them to go to charity.
Mr Ractliffe said he had kept the stones, which could link him to illegal "blood diamonds", because he wanted to protect the reputation of Mr Mandela and his charity.
Stepping down from his role as trustee, Mr Ractliffe apologised for causing "possible reputational risk" to the charity by not informing his colleagues of his receipt of the diamonds, the Nelson Mandela Children's Fund said in a statement.
At the trial, Ms Campbell said she was given some "dirty-looking stones" after a 1997 charity dinner hosted by South Africa's former President Nelson Mandela where Mr Taylor was also a guest.
She said two unidentified men appeared at her room and gave her the stones.
She told the court she did not have proof they came from Mr Taylor and had given them to Mr Ractliffe because she wanted the stones to go to charity.
"Naomi suggested they could be of some benefit to the Nelson Mandela Children's Fund - but I told her I would not involve the NMCF in anything that could possibly be illegal," Mr Ractliffe said in a statement two weeks ago.
He said he took the diamonds as he thought it might be illegal for her to take them out of the country.
"In the end I decided I should just keep them," he added.
Mr Taylor is accused of using illegally mined diamonds to secure weapons for Sierra Leone's RUF rebels during the 1991-2001 civil war - a charge he denies.
Prosecutors say that from his seat of power in Liberia, Mr Taylor also trained and commanded the rebels.
The rebels were notoriously brutal, frequently hacking off the hands and legs of civilians.