A High Court judge has blasted a top City of London law firm for charging nine years’ worth of man hours on a five-day trial over BlackBerry patents, in a judgment that will fuel the growing controversy over lawyers’ billing practices.

Lawyers from Allen & Overy racked up nearly £5.2m in costs representing Research in Motion, the maker of the popular BlackBerry device, in a dispute with Visto, a US-based wireless technology company.