Alexei Navalny, Russia's most prominent opposition leader, is grappling with a mystery ailment in jail that could be some sort of slow acting poison, as he has lost 8 kg in weight in just over two weeks, his spokeswoman Kira Yarmysh said.

"We do not exclude that at this very time Alexei Navalny is being slowly poisoned, being killed slowly so that it attracts less attention," Yarmysh said in a post on Twitter.

"He is being held in a punishment cell with acute pain without medical help," she said.

A former lawyer who rose to prominence more than a decade ago by lampooning President Vladimir Putin's elite and voicing allegations of corruption on a vast scale, Navalny has long forecast Russia could face seismic political turmoil.

In 2020, he survived an apparent attempt to poison him during a flight in Siberia, with what Western laboratory tests determined was a nerve agent. Russia denies that the state tried to kill him.

He was treated for that poisoning in Germany but voluntarily returned to Russia in 2021, where he was arrested on arrival and jailed in a fraud case he calls politically motivated.

Yarmysh said that due to Navalny's mystery ailment, emergency services were called overnight on Friday to Saturday to the maximum-security IK-6 penal colony at Melekhovo, about 250 km (115 miles) east of Moscow, where Navalny is being held.