By Steve Slater
LONDON, June 27 (Reuters) - Barclays Plc said it is bringing in outside help to speed up an investigation into alleged misconduct in its "dark pool" operations, which have rekindled concerns that the cost of past problems at its investment bank will rise.
More than 2 billion pounds ($3.4 billion) was wiped off Barclays' market value on Thursday after New York's attorney general filed a lawsuit against the British bank, accusing it of trying to grab extra profits from its alternative trading system while promising to get the best possible prices for customers.
The shares stabilised on Friday and were up 0.6 percent at 216.2 pence by 1200 GMT after tumbling 6.5 percent to a 19-month low on Thursday.
Dark pools allow institutional investors to trade large blocks of shares anonymously. Prices are posted only after deals are done, and the pools were created so that investors do not suffer a disadvantage by signalling their big orders.
But ever-larger volumes of trades have been shunted into them and critics say the opacity of the markets may be resulting in more investors getting ripped off.
Barclays' dark pool business, called LX, originally belonged to Lehman Brothers, the investment bank that collapsed in 2008. Barclays subsequently bought Lehman's U.S. business.
The business under scrutiny generates relatively modest revenues for the bank.
The 31-page summons issued by the attorney general said internal Barclays documents valued the growth opportunity from pushing more orders into its dark pool at between $37 million and $50 million per year.
Total revenues for the dark pools business may be $100 million to $200 million, industry sources and analysts estimated, out of $4.6 billion in equities revenues last year.
Analysts said the hit to its shares reflects broader concern that customers may leave Barclays, that Chief Executive Antony Jenkins will struggle to turn around the bank's culture as quickly as he needs to, and the threat of higher-than-expected litigation costs on a range of issues.
"The complaint reminds investors of the litigation burden that faces the sector and Barclays," said Michael Helsby, analyst at Bank of America Merrill Lynch.
Barclays has been hit by series of scandals in recent years, including its role in the rigging of the Libor interest rate which cost chief executive Bob Diamond his job in 2012, leading to huge fines and legal bills.
Helsby raised his estimate of possible litigation costs for Barclays to 7.5 billion pounds over the next three years, from 2.4 billion, and cut his price target on the stock to 285p from 350p.