Why is High Frequency Trading legal?

Michael Lewis's new book Flash Boys is the book that's got just about everybody on Wall Street talking and arguing

The row's been sparked by fresh claims from the former New York bond trader Michael Lewis.

He's man who lifted the lid on Wall Street excess in the 1980s, with his book Liar's Poker.

In Flash Boys, he claims traders who use computers to buy and sell shares and other investments in lightning-fast time have taken over Wall Street. And everyone else is losing out as a result.

He's not the only one who's concerned. New York's attorney General has called for more regulation of high frequency traders.

and the FBI has been openly calling for whistle blowers to come forward with any information about those traders gaining unfair - and illegal - advantage in the market.

The furore has led one trading firm, called Virtu Financial, to delay a planned New York flotation.

So... what exactly IS high frequency trading?

Our economics correspondent Andrew Walker explains

To get a better idea of whether the world really should be worried about high frequency trading

we turn to Richard Payne, professor of finance at the Cass Business School here in London.