Mikhail Kasyanov said on Monday he was ready to give evidence as a witness in a new trial of Yukos founder Mikhail Khodorkovsky.
Khodorkovsky and his business partner Platon Lebedev, who have been serving eight-year sentences for fraud and tax evasion since 2006, are facing fresh embezzlement and money laundering charges.
The new charges deal with the theft of government shares and the laundering of money earned from illegal oil sales, in total worth about $50 billion. If convicted, the two could see their jail terms extended by 21 years.
"If the court summons me as a witness, I will attend and answer any questions that are put to me," Kasyanov said in an interview with Ekho Moskvy radio.
He added that he was also ready to testify at the European Court of Human Rights.
"I must tell the truth and give such testimony," he said.