Yukos, Russia's largest oil company, was hit by a severe downgrade on Tuesday from Standard & Poor's and Moody's Investors Service to a near default level over concerns that the company may lose control over its cash-generating subsidiaries. Yukos' share price fell 5 per cent, taking losses since the start of last week to more than 14 per cent. However some brokers claimed there has been some active buying of Yukos shares over the past few days which had prevented the share price falling further. The downgrade by Moody's from Ba1 to B1 of one of the country's most liquid stocks is a blow for investors in the Russian market. S&P cut its rating from BB- to CCC. "Yukos is seen as a proxy and a benchmark for the Russian market as a whole," said Eric Kraus, strategist with Sovlink, a Moscow-based brokerage. The move follows a ruling by a Russian court banning Yukos from selling or transferring any of its assets in connection with a $3.5bn tax claim brought by Russian authorities at the end of last year. Yukos is challenging the claim. However S&P's said that if Yukos loses the tax case, it "may find itself under threat of forced sale of core assets, insolvency procedures, or effective nationalisation of its assets". "There is also mounting political pressure on the company, which might weaken Yukos' position in the tax case," said Elena Anankina, S&P's credit analyst. Yukos has been under attack from the authorities since the arrest of its core shareholders, including Mikhail Khodorkovsky, a former chief executive, last year. The tax evasion hearings are expected to start next month and could coincide with the trial of Mr Khodorkovsky in June. S&P also downgraded Sibneft, an oil company 92 per cent owned by Yukos. The two companies had agreed to merge but Sibneft's core shareholders asked for the merger to be reversed following Mr Khodorkovsky's arrest. "The downgrade and the CreditWatch status on Sibneft reflect increasing uncertainties about the potential reversal of the acquisition of Sibneft by Yukos and, consequently, about Sibneft's shareholding structure and financial policy," said Ms Anankina. The rising pressure on Yukos has also put its merger with Sibneft, controlled by Roman Abramovich, into question. The companies agreed to merge last year, but following the arrest of Mr Khodorkovsky, Sibneft asked to reverse the merger.