Small stockbrokers could derail the proposed merger of the London and Frankfurt stock exchanges in a row over the cost of switching IT systems.
A merged exchange, to be called iX, would account for more than half the traded volume of European shares.
However, proposals to standardise the technology on Frankfurt's Xetra electronic trading system have provoked anger among London brokers who in 1996 paid £1bn to implement the Sets electronic trading system.
Andersen Consulting was called in to develop and operate Sets after the debacle over Taurus, the London Stock Exchange's first attempt to build an electronic market in the early 1990s. Taurus cost the stock exchange £75m and City firms a further £300m.
Andersen also designed Frankfurt's Xetra system, which runs on dedicated terminals, unlike Sets, which uses open architecture, and is run by the exchange's in-house team.
Angela Knight, chief executive of the Association of Private Client Investment Managers & Stockbrokers, said, "To be fully involved in these new developments will incur significant costs - who will pay?"
The London Stock Exchange needs a 75% vote to carry the merger, but Knight's association has a third of the votes.