Maria Eismont told reporters that the conditions of the transfer would amount to torture for Kara-Murza, 42, who suffers from a serious nerve condition.
Convicts are normally moved across Russia's vast distances in a series of railway journeys with stops at prisons en route. Eismont said transferring Kara-Murza from Omsk to Moscow was likely to take at least three weeks, during which time he would have no contact with his family or lawyers.
Russian media reported that the Supreme Court had ordered that Kara-Murza be brought to Moscow for his appeal because his case involved state secrets and could not be discussed by video link from his Siberian prison.
Kara-Murza, who has Russian and British citizenship, has condemned Russia's war in Ukraine and lobbied for Western sanctions against Moscow. He was sentenced last year after what he described as a show trial like those under Stalin in the 1930s.
His wife Evgenia has voiced fears for his life following the death of Alexei Navalny, Russia's best-known opposition figure, in an Arctic penal colony in February.
(Reporting by Mark Trevelyan)
