The International Swaps and Derivatives Association (ISDA) may have acted with investment banks to block exchanges from entering the credit derivatives market, EU antitrust regulators said as they expanded their investigation into the sector.

The investigation into credit default derivatives is one of several by the European Commission into the financial services industry. Opacity in derivatives played a central role in the 2007-09 financial crisis, leading to calls for action.

The Commission, which acts as EU competition watchdog, opened its CDS investigation in April 2011, listing 16 banks and financial data company Markit.

"The Commission's inquiry found preliminary indications that ISDA may have been involved in a coordinated effort of investment banks to delay or prevent exchanges from entering the credit derivatives business," the EU executive said in a statement on Tuesday.

ISDA said it was confident it had not breached any EU competition rules.

"ISDA is aware that it has been made subject to these proceedings," said the industry body, which represents the world's biggest banks that trade financial derivatives.

"ISDA is confident that it has acted properly at all times and has not infringed EU competition rules. ISDA is cooperating fully with regulatory authorities," it added.

The Commission said in 2011 that banks under investigation included JP Morgan, Bank of America Merrill Lynch , Goldman Sachs, Deutsche Bank, Citigroup, Barclays and BNP Paribas.

Societe Generale, Commerzbank, Credit Suisse First Boston, HSBC, Morgan Stanley , Royal Bank of Scotland, UBS, Wells Fargo Bank/Wachovia and Credit Agricole were also named.

Credit default swaps are a form of tradeable insurance against the non-payment of debt, such as a corporate or government bond. At the height of the crisis, their price climbed for countries such as Greece, exacerbating borrowing difficulties.