The London Stock Exchange has already lost a quarter of its near-monopoly in London business in equities to rival exchanges, figures seen by The Times show.

Tracking reliable market share figures for trading in London equities is notoriously difficult, because each of the various participants tends to use a different measure.

The main independent figures come from Thomson Reuters. However, these include business done on Boat, a joint venture between banks that merely records trade bargains. If the Boat total is removed and the other market shares are rescaled, it is clear that the LSE still has about 75 per cent. However, Chi-X, the second contender, ultimately owned by Nomura, the Japanese bank, has managed to build a share of 15 per cent since it entered the market in March 2007.

Two other recent entrants – the American-owned Nasdaq OMX and BATS – fail to register because they have only just started. The Thomson Reuters figures still include trades that took place off-book but were reported through the various exchanges, which is less lucrative for the exchanges than actual share-dealing. Figures from the LSE itself, however, suggest that it has lost more than a fifth of the business in FTSE 100 stocks.

The amount of its market share that the LSE will lose to new entrants and to Plus Markets, the revived Ofex market, has been a vexed issue in the City. Dame Clara Furse, the LSE chief executive, has always insisted that, with market volumes rising fast in recent years, there is more of the cake to share around and her exchange’s actual volumes should not decline.

Yet there have been signs of a slackening off in the value of trade done on the LSE in recent months.

An LSE spokesman said that the rising number of firms using computerised algorithmic trading coming into the market would continue to boost volumes. Six of these have arrived this year.

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