New York's attorney general has accused the bank of "a disturbing disregard for its investors".
Barclays is being fined for manipulating the price of gold, after the Libor rate-fixing scandal and PPI insurance mis-selling.
Barclays faces another scandal after it was fined £26 million by the City watchdog over failings linked to the fixing of the price of gold.
Barclays has announced its adjusted pre-tax profits are down 7% for the first half of the year to £3,35 million.
The drop was largely down to currency movements and a fall in profitability for its investment arm, the bank said.
British bank Barclays is taking allegations "very seriously" that it misled large institutional investors and other clients in the United States by falsely telling them it was taking measures to protect them from predatory high-frequency traders, it said today.
A Barclays spokesman said: "We take these allegations very seriously.
"Barclays has been co-operating with the New York Attorney General and the SEC (Securites and Exchange Commission) and has been examining this matter internally. The integrity of the markets is a top priority of Barclays."
Barclays bank has been accused of misleading large institutional investors by duping them into buying protection measures from predatory high-frequency traders, New York's attorney general said.
The allegations against the British banking and financial services firm were contained in a securities fraud lawsuit that Eric Schneiderman announced at a Manhattan news conference.
The NY attorney general accused the bank of "a flagrant pattern of fraud, deception and dishonesty with Barclays clients and the investing public," in a complaint filed in the Supreme Court.
Barclays stands accused of deceiving investors about its dark pool, which allows bank clients to trade blocks of shares while keeping prices private.
Barclays spokesman Mark Lane said the bank, headquartered in London, was co-operating as "integrity of the the market is a top priority."
The Financial Conduct Authority (FCA) has said Barclays had failed to manage conflicts of interest between itself and its customers in relation to the way the price of gold is set, between 2004 and 2013.
According to the FCA, the fine surrounds the actions of Barclays trader Daniel James Plunkett:
- Mr Plunkett, was a trader on the Barclays precious metals desk on June 28 2012.
- Mr Plunkett "exploited the weaknesses in Barclays' systems and controls to seek to influence that day's 3pm gold fixing, the FCA said.
- As a result, Barclays did not have to make a 3.9 million US dollar (£2.3 million) payment to a customer, though it later compensated the customer in full.
- Mr Plunkett's actions boosted his own trading book by 1.75 million US dollars (£1.04 million), the FCA said.
- Mr Plunkett had agreed to settle at an early stage of the probe, meaning they each qualified for a 30% discount to their fines.
- The watchdog has fined him £95,600 and banned him from the industry.
The gold-fixing failings by Barclays has once again tarnished the industry's reputation, the Financial Conduct Authority said, after it fined the bank £26 million.
Its announcement focused on the behaviour of Daniel James Plunkett, who was a trader on the Barclays precious metals desk, on June 28 2012.
– Tracey McDermott, Financial Conduct Authority
Traders who might be tempted to exploit their clients for a quick buck should be in no doubt - such behaviour will cost you your reputation and your livelihood.
Barclays' failure to identify and manage the risks in its business was extremely disappointing. Plunkett's actions came the day after the publication of our Libor and Euribor action against Barclays.
Firms should be in no doubt that the spotlight will remain on wholesale conduct and we will hold them to account if they fail to meet our standards.
Barclays has been fined £26 million by the Financial Conduct Authority over failings in relation to the fixing of the price of gold.
The chief executive of the Barclays Group has rejected claims that morale has sunk within the bank over the announcement it is to cut 7,000 jobs, arguing that it "would not damage" the organisation "at all".
Antony Jenkins told ITV News: "I don't think it is going to damage Barclays at all. I think our colleagues understand what we are trying to do strategically, and the feedback that I have been receiving all through today is they are excited about the direction of the bank.
"Of course we understand that the people who are directly affected by this will be concerned, and of course we are going to treat them with care and respect."
He added: "This is about positioning Barclays so it can succeed and win in the new world."