Project Turquoise, the share trading system backed by a group of the world's largest investment banks, will allow transactions both on-exchange as well as in "dark liquidity" pools, where firms offer to buy and sell large blocks of shares away from public sight.

Speaking at an Exchange Forum conference in London, Phillip Hylander, co-head of European equities at Goldman Sachs (NYSE: GS - News) , set out for the first time the broad outlines of the business model that has promised to bring more competition to European share trading.

"Turquoise is to be a hybrid that will incorporate a public order book and a non-public order book," Mr Hylander told the conference. "Turquoise will be an aggregator of 'dark pools'."

Plans for Turquoise suggest that its backers are going beyond their initial aim of providing exchange-type services at lower prices than existing providers.

They are also seeking to cater for the large lot sizes that banks will be allowed to fill internally from their own order books following new European rules, known as Mifid, which take effect in November.

Banks say that although exchanges have become efficient fulfillers of smaller orders, larger orders that may throw prices against the customer placing the order are harder to fill.

Separately, shares in OMX, the Scandinavian exchanges and technology group, were suspended on Thursday amid speculation that Nasdaq (NASDAQ: NDAQ - News) , which has expressed interest in buying it, is set to announce an offer.

Swedish media reports said OMX was talking to Nasdaq, the second-largest US stock exchange, which has been frustrated in its efforts to buy the London Stock Exchange, in which it is now the largest shareholder.

Nasdaq said it did not comment on market rumours. Shares in Nasdaq advanced 4.6 per cent to $34.72 in the late afternoon in New York.