Naomi Campbell testifies on Thursday at the war crimes trial of former Liberian leader Charles Taylor, who prosecutors say gave a "blood diamond" to the British supermodel.
The prosecution at the Special Court for Sierra Leone summoned Ms. Campbell to back their allegations that Mr. Taylor received diamonds from rebels in Sierra Leone, which they say he then used to buy weapons during a 1997 trip to South Africa.
Defence lawyers for Mr. Taylor have sought to delay the testimony, arguing the ex-Liberian president's right to a fair trial is being denied because they have not seen the full intended statement by Ms. Campbell. The court was expected to rule later on Wednesday on the request.
Mr. Taylor - accused of being given the diamonds by rebels a month before his trip to South Africa - is said by prosecutors to have given Ms. Campbell a large rough cut diamond after a dinner hosted by Nelson Mandela. Mr. Taylor has said this is "nonsense."
"The timing of this dinner is important because if this story ... is true, it places Taylor in possession of at least one rough-cut diamond the month after the rebels came to see him, and the month before the junta received a large shipment of weapons in October," said Open Society Justice Initiative, a human rights organization monitoring the Taylor trial.
Ms. Campbell's testimony has sparked intense media publicity, with more than 200 journalists hoping to attend. But Ms. Campbell, an unwilling participant, won a court ruling banning photos of her entering or exiting the court on concerns over her security.
She will still be filmed while giving testimony, however.
On trial in The Hague, Mr. Taylor denies all 11 charges of instigating murder, rape, mutilation, sexual slavery and conscription of child soldiers during wars in Liberia and Sierra Leone in which more than 250,000 people were killed.
One of Ms. Campbell's London-based lawyers, Gideon Benaim, declined to comment. "Ms. Campbell will give her testimony in the Hague on Thursday," he said.
Prosecutors have said Ms. Campbell is in a position to provide material evidence and will also call modelling agent Carole White and U.S. actress Mia Farrow to the stand on Monday.
The prosecution said Ms. White heard Mr. Taylor say he was going to give Ms. Campbell a diamond and was there when Ms. Campbell received it, while Ms. Farrow attended the reception where Ms. Campbell met Mr. Taylor and was told later by Ms. Campbell about the gift.
The court has granted one of Ms. Campbell's lawyers a limited right to intervene on whether to allow questions for Ms. Campbell if she could incriminate herself by answering them.
"Direct examination will be very brief. The prosecution has no idea of the length of cross examination," prosecutor Brenda Hollis said. "Ms Campbell's testimony will provide the judges with another piece of the overall picture."
Mr. Taylor's lead defence lawyer Courtenay Griffiths has argued, however, that Ms. Campbell's anticipated evidence is "entirely based on conjecture", stressing in court filings Ms. Campbell has denied publicly ever receiving a diamond or diamonds from Mr. Taylor.
Ms. Campbell had also expressed concern about her safety and initially refused to testify, prompting the court to subpoena her. A subpoena is a legal measure used to gain testimony from an unwilling witness on the threat of a fine or imprisonment.