The world's largest banks could see more than $200 billion more in additional write downs, depending on the outcome for troubled bond insurers.

UBS Analyst Philip finch told clients in a note today that the write down problems are expanding beyond troubles related to subprime loans.

The two largest insurers, MBIA, Inc. and Ambac Financial Group Inc., are at risk of losing their AAA credit ratings, which they depend on for getting new clients.

Finch said that on top of $152 billion in reported losses there could be up to $203 billion in additional write-downs. Banks would have to write down securities protected by the so-called monoline insurers if their financial situation worsens.

Write downs related to complex financial instruments known as collateralized debt obligations could rise by $120 billion, according to Finch. There could be up to $50 billion related to structured investment vehicles.

Commercial mortgage-backed securities and write downs in leveraged buyouts could reach $18 billion and $15 billion respectively, he said.

Other troubles include government regulations which could cut profits at banks by 5.3 percent, and additional fund raising efforts, he added.