Trying to select a simple definition for prime brokers and to identify precisely what it was they did, has historically been complicated by the breadth, range and diversity of their activities. In retrospect, that was a good problem to have. Far worse is the situation prime brokers now face – trying to identify what will be left for them to do in the future.

Securities' lending - sourcing hard-to-find securities and lending them on - made up a good portion of their business. But swathes of lenders have now exited the market, either because they have been advised against lending by consultants or because they have been discouraged by media criticism.