Seidman is facing a $521m (£258.7m) lawsuit for failing to spot a fraud during its audit of factoring group Bankest. But Miami-Dade Circuit Judge Jose Rodriguez, who heard the case, has already ruled that Banco Espirito Santo could show no evidence to establish its claim against BDO International.

The judgment has meant BDO figures around the world are more relaxed about the US lawsuit than had been expected. Other firms have in the past faced international lawsuits, such as on Parmalat, following negligence suits relating to one particular national firm.

Seidman has an annual turnover of $589m, meaning the Bankest damages could overwhelm the firm. But BDO sources said this week that the case would almost certainly have a long trip through the US courts.

Seidman was scheduled to file an appeal last week on the action, with sources suggesting that the appeals could take two to three years to work through the US legal system.

The firm is set to argue that the punitive damages awarded are inconsistent with Florida law, a source said. The firm is confident that it will emerge unscathed from the litigation.

Seidman, which audited Bankest for seven years, was to file its appeal against punitive damages last week and has defended itself vigorously from the start.

Stoy Hayward, which relies on Seidman for referrals and US work, has expressed its confidence in Seidman's ability: 'BDO Seidman remains capable of handling work for any of our clients operating in the USA and is a key part of the BDO International global network.'