Investment Technology, an American group, and Societe Generale, the French bank, are behind this latest share trading initiative, which has functioned with growing success in the US for 10 years; it traded 2.6 billion shares there in the first half of this year.
The two have formed a fifty-fifty joint venture called ITG Europe which its chief executive, Alasdair Haynes, describes as a new type of stockbroker; in effect it is an electronic execution-only broker.
It is aimed at institutional investors as well as stockbrokers. But private investors will not be allowed to use the system directly; they will have to go through their broker.
Posit (Portfolio System for Institutional Trading), due to be launched in November, accepts orders telephoned or faxed to its trading desk or pumped directly into its system through an electronic link.
Twice a day, at 11 am and 3 pm, Posit will compare all orders, matching the maximum possible. Trades will be priced at the mid stock market price ruling at 11am and 3pm. Any unmatched deals will be completed if the City's new broker can manage to do so outside the system. It will be possible to leave a deal, with price limits, in the Posit box.
Posit will offer a trading facility for most shares - the 125 blue chips on the order book through to tertiary stocks in the lower, often neglected, reaches of the market. It could possibly help to alleviate the difficulty of dealing in the shares of smaller companies.
Orders are held in the Posit system; so the identity of the buyer and seller and details of orders are kept secret. There are also, it would appear, distinct cost advantages.
So has yet another rival to the Stock Exchange emerged? There must be a possibility that Posit will attract trade from the Stock Exchange's much criticised order book, where rogue trades occur with monotonous regularity, distorting prices and the Footsie calculation.
It is estimated that only 30 per cent of available trading goes through the order book. This is a humiliating performance for such an expensive, highly hyped system, which is a year old next month.
But there is an argument that Posit complements order-driven trading and could actually benefit the Stock Exchange. The suggestion is the new system will not attract much, if any, trade from the order book. But it will draw heavily on the 70 per cent of business the order book has failed to attract.
It is a crushing indictment that so much trading is still directed at the old market-makers who, freed from the obligation to trade in shares at signalled prices and sizes, are having a field day.
It seems that dealing in the dark, admittedly with people you may know and presumably trust, is preferable for many investors to encountering the perils of the order book.
Clearly Posit could be a compelling attraction for those who have decided to ignore the order book. And there is also its possible appeal for trading in small companies. The Stock Exchange should be worried.
ITG Europe will be a Stock Exchange member, as is JP Jenkins which runs the fringe Ofex share market.
It is setting up shop at a time when Tradepoint, which did see itself as a rival, albeit a modest one, to the Stock Exchange, is going through a difficult period, and Easdaq, the European version of Nasdaq, has failed to make much impression.
ITG Europe, based at Dublin for tax reasons, plans to take Posit to Frankfurt next year and intends to cover the main European stock markets within two years.
Mr Haynes, who is 38, is a former derivatives trader. Before joining ITG Europe he was in charge of global derivative trading at HSBC.
He has made around 100 Posit presentations to City houses and has encountered, he says, enthusiasm for this latest share trading import from the US.
Diageo, the spirits behemoth leads this week's profits parade but is unlikely to engender much enthusiasm - indeed sobriety may be the order of the day. With the Asian crisis, a downturn in Latin America and the strong pound, profits will be down, say, pounds 1.85bn against pounds 1.93bn.
Tesco, as another of those monotonous price wars breaks out among the supermarket brigade, offers interim figures. So far Tesco has not joined in the new price friction; it will almost certainly do so. The market expects profits of around pounds 370m, up from pounds 350m.
PizzaExpress should turn in another sizzling display with year's profits around pounds 23m against pounds 16.2m, and Avis Europe should be set for pounds 30m at the halfway mark against pounds 22.3m.
Builders continue their profits season and Barratt Developments should be on line for year's profits of pounds 92m against pounds 70.1m. Kier, which acquired house-builder Bellwinch, is expected to offer final figures of pounds 11.3m, up from pounds 8.6m.
On the building materials front, Tarmac is likely to produce pounds 46m against pounds 38.6m.
Eurotunnel could check in with an interim trading profit - pounds 170m against a pounds 323m loss.