In a statement to AFP, state communications watchdog Roskomnadzor said that access to the website votesmart.appspot.com had been blocked in Russia “because it is being used to continue the work … of an extremist organisation”.

Parliamentary elections are scheduled to take place on Sept 17 to Sept 19, with nearly all vocal Kremlin critics including Navalny’s allies barred from running.

Navalny, 45, has this year seen his organisations declared “extremist” and banned, while his top aides have fled the country.

After barely surviving a poisoning with nerve agent Novichok last summer, Navalny was jailed in February in what supporters say is punishment for seeking to challenge President Vladimir Putin’s two-decade hold on power.

Roskomnadzor earlier barred access to dozens of websites linked to Navalny, including his main site navalny.com.

Last week the media regulator also urged Google and Apple to remove an app dedicated to Navalny’s “Smart Voting” campaign from their app stores, but they have yet to respond.

The “Smart Voting” tactic has led the increasingly unpopular United Russia party, currently polling at less than 30%, to lose a number of seats in recent local elections.