MOSCOW, September 28 (RIA Novosti) - Former Russian Prime Minister 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.

Later on Monday, Moscow's Khamovniki Borough Court will start the cross examination of witnesses in the second case against Khodorkovsky and Lebedev.

There are 250 people on the prosecution witness list and 478 for the defense, including Prime Minister Vladimir Putin and Finance Minister Alexei Kudrin.

However, Judge Viktor Danilkin has so far refused to approve the list of defense witnesses, Khodorkovsky's counsel said.

Lawyers acting for the businessmen have called their clients political prisoners, and say the new charges are "disgraceful" and unsubstantiated.

The Russian justice minister said in July the Strasbourg-based ECHR would consider a complaint from Khodorkovsky in the fall.

Three appeals have been filed with the Strasbourg court against the trial in the first case and the investigation into the second case, which is ongoing.

Once Russia's largest oil producer, Yukos collapsed after charges of tax evasion led to the company being broken up and sold off to meet debts. The bulk of the company's assets were bought up by state-run oil company Rosneft.