Russian investigators have not yet established the cause of Alexei Navalny's death and it is unclear how long it will take for official conclusions to be made, his mother was told, Navalny's spokeswoman said on Monday.

Navalny, a 47-year-old former lawyer, fell unconscious and died on Friday after a walk at the "Polar Wolf" penal colony in Kharp, about 1,900 km (1,200 miles) northeast of Moscow, where he was serving a three-decade sentence, the prison service said.

Navalny's mother, Lyudmila, was told on Saturday at the prison colony that he had perished from "sudden death syndrome", a vague term for different hearth conditions that end in death, according to Navalny's team.

Navalny's spokeswoman, Kira Yarmysh, said that his 69-year-old mother, and lawyers were told that the official verification of the cause of death had been extended and that it was unclear how long it would take.

"The cause of death is 'undetermined'," Yarmysh said, adding that the Russian authorities were lying and stalling.

His mother and lawyers were not allowed into the morgue on Monday in the Arctic town near the prison colony where the authorities said he dropped dead, Yarmysh said.

"Asked if Alexei's body was there, the staff did not answer," said Yarmysh.

The death of Navalny, a former lawyer, robs the disparate Russian opposition of its most charismatic and courageous leader as President Vladimir Putin prepares for an election that will keep him in power until at least 2030.