Misconduct allegations surrounding foreign exchange trading are "every bit as bad" as the Libor rate-rigging scandal, the head of the City regulator told MPs today.
At least 10 banks have been drawn into a major investigation launched last autumn by the Financial Conduct Authority (FCA), the Treasury Select Committee was told.
FCA chief executive Martin Wheatley also said it was investigating a range of benchmark rates operating in London.
Banks have been fined billions of pounds by regulators in the UK, Europe and the US for trying to fix such rates, including Libor, and Barclays chief executive Bob Diamond quit after the scandal emerged in 2012.
Criminal action has also been launched by the Serious Fraud Office.
Last October, the FCA said it had joined other international regulators in scrutinising firms over potential manipulation of the £3 trillion a day forex market. Mr Wheatley said: "The allegations are every bit as bad as they have been with Libor."
He said that, given the allegations, there was a lack of trust in the way such rates were fixed - though the FCA was putting in place new standards.
Mr Wheatley said 10 banks had come forward with information in the light of the forex probe, though the regulator itself had not named those involved.
He said further investigations had not been made public, but told MPs: "There are a number of other benchmarks that operate in London that we are investigating because of concerns that are being raised with us."
Benchmark rates are used for hundreds of trillions of dollars-worth of loans and transactions around the world and are calculated using submissions from panels of banks about the rates at which they believe they can borrow every day.
Yesterday, the FCA revealed that it had used new powers for the first time to issue warning to two bankers over alleged Libor rigging.
Mr Wheatley's evidence came during a wide-ranging session before MPs in which he was also grilled over bonuses at Lloyds, the appointment of former Co-op bank chairman Paul Flowers and allegations against Royal Bank of Scotland.
The FCA boss agreed that bonuses for Lloyds senior managers should be cut in the wake of a record £28 million fine over incentive schemes that drove mis-selling.
He told MPs the body was still going through pay-outs for this year so he could not comment in detail on how this should be done.
But he agreed with MP Jesse Norman when asked whether there ought to be a "meaningful deduction from the bonus pool".
Mr Wheatley said: "The whole concept is that those people involved in the decision-making should suffer the consequences of our subsequent enforcement action."
Lloyds was fined in December over schemes that rewarded staff with "champagne bonuses" and put advisers under pressure to hit sales targets or face demotion.
The FCA probe focused on sales of investment and insurance protection products between January 2010 and March 2012, a period when more than one million such products were sold to over 690,000 customers.
In the worst case, one adviser sold insurance products to himself, his wife and a colleague to save himself from being demoted.
Mr Wheatley told MPs that the incentives "resulted in a sales-driven culture which did do damage to Lloyds customers".
The probe centred on a period after many banks had embarked on reforms after the financial crisis, and Mr Wheatley said: "It was quite surprising from our point of view that lessons had not been learned."
He was also questioned by MPs over the appointment of Paul Flowers at the troubled Co-op bank and told them it could not happen again.
Concerns have been raised over how regulators could have approved him given he had no significant banking experience.
The lender had to be rescued by bondholders including US hedge funds after a £1.5 billion black hole in its finances was discovered. Mr Flowers was later exposed in a newspaper drugs sting.
Mr Wheatley told MPs that systems had changed so that someone with a "demonstrable lack of banking experience" would not be able to hold such a role.
He said: "I can say with some confidence that the process that allowed it to happen in the past has now been changed and would not allow that to happen again."
On RBS, Mr Wheatley admitted that the regulator had had information about its alleged unscrupulous treatment of small businesses since before a major report by Government adviser Lawrence Tomlinson in November.
Mr Tomlinson accused the state-backed lender of driving firms to collapse in order to profit from their property assets.
Mr Wheatley said: "We had had some information prior to that, that we had been responding to and gathering information on."
MPs also heard that regulators held discussions with banks on the subject of a deadline for claims over mis-sold payment protection insurance - a scandal which continues to see banks increase the billions of pounds in provisions set aside for redress.
Mr Wheatley said such discussions would have been held many times over the last three years, but that a strong case would have to be needed to limit the rights of consumers in such a way.
On European rules capping banker bonuses, he reiterated his view that they could have "perverse consequences" as they would simply result in bankers getting round the rules by increasing salaries that could not be clawed back.