Popov was rushed to hospital early Sunday, Interfax reported.
He contracted food poisoning during a meal in the eastern Chechen town of Gudermes, a week before Chechnya's Kremlin-sponsored presidential elections are scheduled to take place.
Doctors confirmed Popov was “poisoned by a poison of unknown origin,” AFP reported. Interfax reported earlier that the possibility of an assassination attempt has yet to be ruled out.
Popov last month took on the duty of acting head of Chechnya's pro-Russian administration, as Kadyrov, head of the region’s administration, is running for the office of Chechen president on 5 October.
Kadyrov, who is supported by Russia, is widely expected to be elected after other likely candidates were either barred from running or otherwise removed.
Russia poured troops into Chechnya on October 1, 1999 to suppress a separatist insurgency. Up to 80,000 federal troops are stationed in the southern republic.