But Gavin Casey, the Exchange's chief executive, believes the introduction of Sets has been a success. "It's gone very well ... the feedback we've been getting is very good," he said.
In the month following launch on 20 October, Sets' share of all FTSE trades averaged 36 per cent. But, in recent weeks, Sets has been losing ground. In the three months since launch, Sets' market share averaged just 32 per cent
Sets, although regarded as a technical success, has been subject to criticism from commentators, smaller stockbrokers, and private investors.
The new system has led to illiquidity at the beginning and the end of the day, meaning investors trading at these times risk receiving unfavourable prices for their shares. The illiquidity has also caused "rogue" or unrepresentative closing prices.
Sets has also been criticised for facilitating market abuse. Last year, two JP Morgan traders succeeded in manipulating the FTSE 100, earning their employer a record pounds 350,000 fine.
"There are things we need to work on", admitted Mr Casey yesterday.
The Exchange chief said shorter opening hours were one option, with the Exchange possibly closing at 4.10pm, the same time as LIFFE, the futures and options exchange. Other options include introducing a "closing auction". This could help stamp out rogue closing prices, but would not solve the problem of early morning illiquidity.
Mr Casey confirmed the Exchange was still planning to extend the new order book to cover FTSE 250 stocks. But other, smaller, stocks would not go on to Sets.