All the share trades that are carried out every day on the New York and London share markets and around the world – prices, bid-offer spreads and trade sizes – are visible to every member of the investing public. Right?

Wrong. That is the strange truth behind the emergence of “dark pools” of liquidity – a relatively new venue for the buying and selling of shares. Pre-trade prices – the price at which shares are offered for sale – are not visible to anyone, even the participants in them, and the price at which shares change hands is only revealed after the trade is done. No wonder dark pools have started to attract regulatory scrutiny. Sandy Frucher, a senior executive with Nasdaq OMX, said at a London conference a few weeks ago the big question was whether they would “pass the regulatory smell test”.