The US tech giants have been threatened with an initial financial sanction of up to four million roubles ($55,000), if they fail to restrict access to the app. Last month, the companies were warned by the state regulatory agency Roskomnadzor, they could face "criminal liability" if they were deemed to be taking part in the work of "extremist organisations banned in Russia." In a statement on Tuesday, the regulator said: "Failure to take action is a breach of Russian law … and may also be viewed as interference by the US companies in Russia’s elections."

       Maria Zakharova, a Foreign Ministry spokeswoman, later said that the refusal by US tech giants to delete “illegal content” had become “systemic”.

       The move comes just weeks before parliamentary elections are held in Russia.

       The Kremlin has in recent months launched a crackdown on regime opponents, in a bid to ensure a majority for the increasingly unpopular pro-Putin party, United Russia.

       Alexey Navalny, one of Vladimir Putin's fiercest critics, has been imprisoned and has seen his Foundation for Fighting Corruption branded an "extremist organisation".

       As a result, anyone seen to be promoting the Foundation could face lengthy prison sentences.

       Navalny devised his Smart Voting app as a way to circumnavigate the Kremlin's attempts to bar him from seeking political office.

       READ MORE: Putin accused of ‘orchestrated campaign’ against ‘emotional’ journos

       It allows voters to engage in tactical voting against the so-called ruling party United Russia.

       Tactical voting helped defeat a number of United Russia's candidates in recent local elections.

       Navalny has urged his supporters to download the app in a message from prison.

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       Kira Yarmysh, his spokeswoman, accused Russian authorities of trying to illegally interfere with the elections by trying to block the app.

       In a Twitter post, she wrote: "If something can be called ‘interference in Russian elections’, it is the attempts of Roskomnadzor to block the Smart Voting app.

       “Download them asap.”