Offshore companies created in Panama, the British Virgin Islands and elsewhere can be impenetrable to authorities – and anyone else poking around. That has made them legitimate vehicles for wealth protection and tax planning, but also hideaways for tax dodgers, frauds and worse.

Some of the world's biggest banks, whose clients seek discretion, operate next to the offshore specialists that create and register companies, find "nominee" directors and shareholders to take the true owners' place on forms, and assemble complex, bespoke structures.