Taylor is accused of supporting the rebels in Sierra Leone's 11-year civil war in exchange for diamonds and other natural resources. He denies trading in the so-called "blood diamonds."
Prosecutors at Taylor's trial in The Hague have summoned Campbell to testify later this month about reports that she received diamonds from Taylor during a 1997 reception in South Africa.
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The Outside Organization, a public relations company that represents Campbell, announced on Friday that Campbell would testify. Her appearance is scheduled for July 29.