Lebedev must now stay in jail until end of December
A key ally of arrested Russian tycoon Mikhail Khodorkovsky has had his own period of detention extended until the end of the year.
Platon Lebedev, held since July, will not now be freed until 30 December at the earliest.
He is a close associate of Mr Khodorkovsky, Russia's richest man, who was arrested at gunpoint on Saturday.
The detention of Mr Khodorkovsky sent shock waves through Russian financial markets on Monday, although prices steadied on Tuesday.
He is under investigation for alleged tax evasion and fraud in his giant oil firm, Yukos.
Mr Lebedev is a major shareholder in Yukos. A previous court order had ordered him to be detained no longer than 30 October.
He is accused of theft in an affair dating back to a privatisation deal in the 1990s.
Critics say both men are being held for political rather than criminal reasons.
"We did not expect any other decision because they are not playing by the rules," Mr Lebedev's spokesman, Yuri Kotler, told Reuters news agency.
Khodorkovsky also faces months behind bars
"The prosecutor general's office again said that Lebedev represented a public danger and would try to escape.
"The example of Mikhail Khodorkovsky clearly shows that businessmen do not run away even when they are given warnings."
Mr Khodorkovsky is being held in a high-security detention centre in Moscow.
He can also be detained until 30 December.
As markets steadied on Tuesday, Russian Prime Minister Mikhail Kasyanov appealed for calm.
"Yesterday, the situation was tense. But there is no need to panic and we must not panic," Kasyanov told reporters. "We must adopt measures to stabilise Russian financial markets."
The detention of Mr Khokorkovsky has unleashed a wave of criticism against President Vladimir Putin.
Liberal commentators have accused the president of being under the influence of Kremlin hawks, returning Russia to authoritarian rule.
"Society has undergone a shift. There are now more elements of totalitarianism than under (former President Boris) Yeltsin and under the earlier Putin," said Igor Bunin, general director
of the Centre for Political Technologies think-tank.
Khodorkovsky is in high-security centre
Former economy minister Yevgeny Yasin said Mr Putin had dashed the hopes of his former backers.
"Before we fought for the president and against a part of
his (Kremlin) entourage," he was quoted as saying in Gazeta newspaper.
"Now, it seems, he has made his choice.
The time of general affection for the president has gone - now
many are wondering how they can vote for him."