In a sign of the explosion in audit activity since the passage of the Sarbanes-Oxley compliance law, the median fees earned by the world’s top auditing firms shot up by 345 per cent in the five years to 2006, a study said on Wednesday.

The findings came in a study of 100 auditing firms and 1,300 companies by The Corporate Library, a respected US corporate governance research firm which billed it as “the most extensive study of the audit industry ever published”.