Activist investors have sent shockwaves through corporations in recent years, personally targeting directors and executives at some of the world's largest companies. No longer satisfied with operating on the fringes of business, they are now a firm fixture in the boardroom.

Up to a quarter of public companies could be targeted by activist campaigns in the coming years, with directors and executives at those corporations threatened with losing their jobs. The trend, which began in corporate America, has spread to the UK, Europe and Asia, taking in several high profile...