This week the third outside competitor to the London Stock Exchange's near-monopoly in share trading in London will enter the fray.

Dealing at the American-owned Nasdaq OMX will begin in a limited way on Friday, to build up over the next few weeks. Eventually the new market will offer trading in about 700 stocks across Europe.

Nasdaq OMX, an offshoot of the New York market, arrives after Turquoise, a trading platform owned by a consortium of investment banks, was launched fully this month. Chi-X, ultimately owned by Nomura, the Japanese bank, was launched in March 2007 with Dutch and German indices, before adding UK stocks.

A fourth new contender, Bats, is expected to open for business next month. Bats, which was set up in 2005 in Kansas City, Missouri, is reckoned already to have taken more than a tenth of equities trading in the United States. It has taken up residence in the old Robert Fleming offices in Copthall Street for its London operation.

A fifth rival to the LSE, Plus Markets, is the revived Ofex over-the-counter market and will focus on small-capitalisation stocks and those on AIM. On Friday Plus announced a High Court action against the LSE over limits on the number of AIM stocks in which it can offer a market.

Dame Clara Furse, the LSE's chief executive, has insisted that the rivals do not pose a significant threat to its future. Its case was damaged when a computer glitch halted trading for seven hours on September 8. Some analysts have suggested that within five years as much as half the share trading in London may have gravitated to the new platforms.

All the rivals claim their platform is best. Bats is expected to emphasise its technology's reliability. Nasdaq OMX offers a “routing” system, whereby any orders that it cannot satisfy are sent to whichever market is offering the best available prices. Others have claimed extra efficiency or cheaper tariffs.

Nasdaq OMX has indicated that it wants 20 per cent of the London market in five years' time. Bats has talked in terms of 20 per cent by the end of next year. However, actual market shares so far are hard to gauge. Chi-X says it is averaging an 18 to 21per cent share at the end of the day's trading. Turquoise has begun quietly, attracting perhaps 2 per cent at the outset.

The LSE says the recent financial turmoil is encouraging its customers to stick with the market they know.

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