Hearse drivers are refusing to take Russian opposition politician Alexei Navalny's body from the morgue to his funeral on Friday after receiving threats, his allies said.

Since the Kremlin critic's death in prison almost two weeks ago, his team have accused authorities of trying to prevent him from having a dignified public burial.

"What a disgrace. Now the hearse drivers refuse to take Alexei from the morgue," said Ivan Zhdanov, an exiled ally who managed Navalny's Anti-Corruption Foundation, on Thursday.

Navalny's spokeswoman Kira Yarmysh said funeral directors had received threatening calls from "unknown people" warning them not to transport Navalny's body anywhere.

Navalny died on February 16 in one of Russia's toughest prisons in northern Siberia, where he was serving a 19-year sentence on charges widely seen as political retribution for his opposition.

Authorities resisted handing the politician's body to his family for eight days, in what his team said was an attempt to "cover up" official involvement in his death.

Russian authorities said Navalny died of "natural causes" but his team and some Western leaders have accused Putin of being directly responsible.

Details of the funeral and how many mourners will be allowed to attend are still unclear, and there has been no comment from authorities on how it will be managed.