Six years ago, Icap chief executive Michael Spencer persuaded banks to sell him their EBS currency-trading platform. These days, he is trying to persuade the same banks to stay on as customers.

The firm made a series of moves aimed at luring high-speed computer traders and wound up alienating some of its biggest clients, who say the changes put their slower, human traders at a disadvantage. Trading volume is down, and EBS is losing share to smaller competitors.