Accountancy firms look set to be barred from offering expert witness services to many of their largest clients under the US Sarbanes-Oxley Act, as the full scope of the radical anti-fraud act emerges.

Under the act, passed by the US Congress in July amid a wave of corporate scandals, ‘expert services’ are listed as a service that accountancy firms are prohibited from providing to their Securities and Exchange Commission’s (SEC) registered clients.