NEW YORK (Bloomberg) — Banks and securities firms have written down more than 80 percent of sub-prime loan losses and "further ratings impact is likely to be minimal", Fitch Ratings said in a report.

UBS AG, Merrill Lynch and Citigroup Inc. are among lenders that have already been "taken down at least one notch," Fitch analyst Eileen Fahey said.

"We don't think things could get significantly worse for them," she said. "The boom times for investment banking are gone, and it's really going to be an issue of strategy and pricing your risk as close to the market as you can get."

Losses directly related to sub-prime lending may be between $400 billion and $550 billion, the report said. Banks and securities firms, which account for about half the deficit, reported $165.3 billion in losses as of this month, Fitch said.