Three years ago last month, Icap, the world’s largest interdealer-broker, called time on its attempt to create a global, full-service cash equities agency brokerage. The venture was a rare misguided investment pursued by Michael Spencer, Icap’s founder and chief executive, but it continues to provide valuable lessons as banks re-engineer their share-trading franchises.

In defiance of the fallout of Lehman Brothers' collapse, the unit's logic was simple: to fill a perceived gap between embattled bulge-bracket banks and smaller, cash-strapped boutiques. However, the division's rapid growth ultimately led to its downfall, resulting in spiralling costs and pitching Icap into direct competition with the major investment banks, the biggest customers in its core broking businesses.