They have said the model can provide material evidence to support their contention that Taylor lied when he testified that he never possessed rough diamonds.

'Material evidence'

Prosecutors have complained to the judges that they had tried unsuccessfully to contact Campbell several times since June 2009, when they received information that Taylor had given her the diamond.

Taylor, right, has denied all 11 charges related to the wars in Liberia and Sierra Leone [EPA]

"The prosecution has shown that there is at least a good chance that the information to be provided by Ms Campbell would be of material assistance to its case," the court said in a ruling.

It cited Campbell's public statements that she "does not want to be involved in the case".

US actress Mia Farrow and Carole White, Campbell's former agent, have agreed to testify in the same trial.

Prosecutors say White heard Taylor say he was going to give Campbell some diamonds and was there when Campbell received them, while Farrow attended the reception where Campbell met Taylor and was told by Campbell about the gift the next morning.

'Blood diamonds'

Prosecutors accuse Taylor of dealing in so-called "blood diamonds", gem stones mined in a war or conflict zone, the proceeds of which are used to finance an insurgency.
Taylor denies 11 charges of instigating murder, rape, mutilation, sexual slavery and conscription of child soldiers during the wars in Liberia and Sierra Leone.

He is accused of supporting rebels in Sierra Leone's 1991-2002 civil war, which claimed the lives of an estimated 500,000 victims.

In return for helping the rebels, prosecutors say Taylor plundered Sierra Leone of its timber and mineral wealth, including gems that became known as "blood diamonds".

The trial is being held in The Hague for fear of renewed violence if the hearings were conducted in Sierra Leone.

After several false starts, the trial began in earnest in January 2008. Taylor's own testimony took seven months.