Gyevorg Dagnyan said the request to keep the defendant in a pre-trial detention center until December comes from Russian prosecutors.
"The Prosecutor General's Office asked for the term of my client's arrest to be extended by three months, until December 2," the lawyer said.
The Yukos vice-president was arrested April 7 and accused of asset theft and money laundering, charges which he denies.
The lawyer said his client's health has sharply deteriorated and that he must not remain in the pre-trial detention center.
Yukos, once Russia's largest oil company, was declared bankrupt August 1 after three years of litigation with tax authorities over the company's tax arrears.
The Yukos Group, whose founder, Mikhail Khodorkovsky, is serving an eight-year prison term after being convicted for fraud in May 2005, now faces a total of $16.6 billion in claims from creditors, including Rosneft-owned former production unit Yuganskneftegaz ($4.07 bln), the Federal Tax Service ($11.6 bln), Rosneft ($482 mln) and more than 20 other companies.
On Thursday, Russian prosecutors opened a criminal case against former Yukos President Steven Theede and other senior managers.
