Wilson Sonsini Goodrich & Rosati is facing the first test of its succession plan following the news that high-profile chief executive Larry Sonsini is to step down after heading the firm for more than 20 years.

The shake-up sees Sonsini hand the helm of the technology leader to John Roos. Sonsini, a Silicon Valley icon and one of the best-known lawyers in America, is to continue as chairman of the firm.