The Russian opposition leader Alexei Navalny has been convicted of libel in a ruling his lawyer said could potentially lead to the jailing of the anti-corruption blogger who is one of President Vladimir Putin’s most prominent critics.

The verdict added to the pressure on Mr Navalny, who is serving a suspended five-year jail term on a theft conviction he claims was orchestrated by the Kremlin. He also faces a separate theft trial in court later this week. "This is clearly just another attempt to chase me into a corner," Mr Navalny said. He has been under house arrest since March.

The judge found him guilty of libel over a Twitter posting calling a city lawmaker from the ruling United Russia party a drug addict. He fined him 300,000 roubles (£5,000).

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Mr Navalny said he would appeal. He denied guilt, saying he is barred from posting on social media under the terms of his house arrest, though relatives and colleagues do post on his accounts. Libel is not punishable by jail but his lawyer, Vadim Kobzev, said the guilty verdict would open the door to a judge putting him behind bars during his upcoming trial instead of under house arrest.

It could also enable prison authorities to ask a court to remove the suspension of his theft sentence and jail him. Mr Navalny, 37, gained prominence with an internet-based crusade against alleged official corruption and helped lead a wave of street protests against Mr Putin in 2011 and 2012.

He said that he would continue his anti-corruption activity. "If somebody thinks... we will stop publishing investigations, it's not going to happen," he said. The protests were the biggest of Mr Putin’s 14-year rule but failed to prevent him winning a third term in 2012.

Critics claim the legal pressure on Mr Navalny is part of a renewed clampdown on dissent. His house arrest keeps him away from protests and Russian internet providers blocked access to his blog at the order of federal prosecutors. Jailing him would anger Kremlin opponents and would add to Western criticism of Mr Putin at a time when he is embroiled in a showdown with the US and EU over the future of Ukraine.

Reuters