Russia's environmental watchdog yesterday said it had filed a Rbs4.35bn ($180m) pollution suit against Norilsk Nickel, the Russian mining group, in its largest ever environmental complaint against a Russian company. Oleg Mitvol, the head of Rosprirodnadzor, who has led campaigns against resource companies in the past that have ended in their partial takeover by the state, accused the company of failing to stop the escape of untreated waste into rivers. Norilsk, the world's largest nickel miner and the target of rival merger bids, denied the charges and said it was going to appeal against them in court. The suit comes after Norilsk's management launched a series of defensive measures this month aimed at boosting the share price in what looked to be an attempt to fend off a full merger bid by Oleg Deripaska's United Company Rusal. UC Rusal is close to completing the purchase of more than 25 per cent of the company, a step it has said is the first towards a full merger. Last week, however, the company's main shareholder, Interros, said it had received a counterbid from Alisher Usmanov, a Kremlin-friendly metals tycoon who also owns a significant stake in Arsenal football club in the UK. Citigroup said in a research note yesterday: "We are concerned the fine may be a sign of political pressure on Norilsk management."