MOSCOW — Russian prosecutors have filed new charges against Mikhail Khodorkovsky that his lawyer said may add 20 years to the jailed former OAO Yukos Oil Co. billionaire’s sentence.

Khodorkovsky, who is due to be released in 2011, was charged with laundering more than $28 billion and stealing 48 million barrels of oil, the Investigative Committee of the Prosecutor General’s Office said yesterday in a faxed statement. He is currently serving eight years in a Siberian jail for fraud and tax evasion.

“There’s nothing fair about this case,” a lawyer for Khodorkovsky, Robert Amsterdam, said yesterday in a Bloomberg Television interview. “I don’t think they’re even trying to make these new charges look real.”

Khodorkovsky’s defense team has been hopeful of winning his early release since President Medvedev took over in May. The state’s campaign against Khodorkovsky, whose Yukos company was bankrupted and sold to state-owned companies, has come under widespread criticism in the West.

Chancellor Merkel of Germany brought up Khodorkovsky’s case with Mr. Medvedev during his visit to Germany last month. Mr. Medvedev, the handpicked successor of President Putin, has promised to ensure the rule of law in Russia.