TOKYO (Reuters) - The collapse of Japan's Yamaichi Securities came as little surprise and need not spark a financial-system crisis -- unless lenders and investors decide to tar healthy institutions with the same brush, analysts say. Making sure that doesn't happen, however, will require not only quick central bank action to ensure liquidity as promised, but efforts by financial firms to improve disclosure and by regulators to prove their surveillance isn't lax. Yamaichi, the weakest of Japan's "Big Four" brokerages, said that it was seeking Finance Ministry permission to cease operations after failing to resolve a corporate crisis born of eroded capital, a racketeer payoff scandal and new evidence of off-book debts and illegal deals. It was the third failure of a Japanese financial institution this month, following Sanyo Securities Co Ltd's filing under bankruptcy law on November 3 and 10th-ranked commercial bank Hokkaido Takushoku Bank Ltd's decision to transfer operations to another bank just a week ago. Worries over the health of Japan's financial system have escalated in recent weeks. Economists said worries that Yamaichi's collapse in and of itself would prompt a financial system meltdown were overdone. But Yamaichi's collapse has again highlighted persistent doubts about financial disclosure and regulators' ability to uncover -- and make public -- misdeeds at financial firms. Yamaichi had long been rumoured to be involved in illegal "tobashi" deals, in which loss-making portfolios are shifted from one favoured client to another client with a pledge from the brokerage that the paper losses will be covered. Japan's official securities watchdog agency gave the firm a clean bill of health in 1993 and as recently as June company executives denied the firm had done any illegal deals. On Monday, however, Yamaichi said it had more than 158 billion yen ($1.24 billion) in losses from "tobashi deals" dating back six years and added it had told the Finance Ministry of its off-balance-sheet debts, which included the "tobashi", on November 17. Analysts noted that some Japanese financial firms have already moved to improve their disclosure, and said investors and lenders were savvy enough to differentiate between the healthy and the financially distressed.