Mounting unease about tax policy could prompt some companies to leave the UK, the government has been warned. Sir Digby Jones, director-general of the CBI employers' group, said this month: "If we're not careful, companies will say Britain's becoming too difficult and move their headquarters elsewhere."

This kind of talk is usually dismissed as an empty threat. In 2000, for instance, a proposal to tighten tax rules on international companies provoked similar warnings. But, then as now, there was widespread cynicism about the threat: in practice, companies rarely move their headquarters.