Dealing with financial regulations is a bit like finding yourself in a bar brawl – you never know from which direction you are going to get hit next. While all the banks have been ducking and diving to try to avoid the haymakers thrown by Dodd-Frank, Basel III, ring-fencing, “Swiss finishes”, Mifid and the gang, a little-known US law has snuck in and starting kicking everyone on the shins.

Last week's news that the private banking arm of HSBC has stopped offering services to US expatriates will have barely blipped on most people's radar. But the reason for the bank's decisions can be traced to new rules that could have profound implications across the financial markets for years to come.