Russian opposition leader Alexei Navalny, who infamously survived a nerve agent 3 years ago, has reportedly fallen ill while behind bars … and some on his team believe he’s slowly being poisoned to death.

The Vladimir Putin nemesis was met with an ambulance over the weekend due to stomach pains, but his lawyer Vadim Kobzez tweeted that there was no diagnosis given after medics assessed him.

Anna Veduta, Vice President of Navalny’s Anti-Corruption Foundation, told The Associated Press, “We believe he is slowly being administered low doses of poison” due to unidentified pills he’s been given.

Veduta added … Navalny lost 18 pounds in 15 days, and first became ill last week when he was let out of a punishment cell and into a regular one. He’s reportedly been moved back into a punishment cell for 15 more days for unknown reasons.

Kira Yarmysh, Navalny’s spokesperson, echoed the potential poisoning claims … saying, “We do not rule out that all this time in prison he could have been poisoned with something to make his health deteriorate slowly but steadily.”

As you know, the documentary “Navalny” won the Oscar for Best Documentary this year — it followed the freedom fighter and former lawyer pushing back against Vladimir Putin and political corruption in Russia.

One of the frightening moments in the doc showed a near-fatal poisoning with a Soviet-era nerve agent in 2020 … an incident for which Navalny squarely blames Putin.

Navalny returned to Moscow in 2021 and was sentenced to 2 and a half years in prison for violating probation from a 2014 case, and got another 9 years tacked on last year when he was found guilty of fraud and contempt.