Fried Frank Harris Shriver & Jacobson is being pressed to release client information on Deutsche Bank in the latest move by US market regulators to test the limits of attorney/client privilege.

The firm is being quizzed by the California Department of Corporations after 200 e-mails that should have been produced by Deutsche Bank as part of an investigation into Wall Street conflicts of interest were left unaccounted for.

New York's Fried Frank had been responsible for handing over Deutsche Bank's e-mails to the regulator, whose investigation forms part of the high profile Securities and Exchange Commission (SEC) probe into alleged conflicts of interest at 12 investment banks.

A spokeswoman for the Department of Corporations told Legal Week: "We are trying to look at the entire chain of custody for the e-mails, from Deutsche Bank to Fried Frank to the regulators. Fried Frank was part of that [chain] so as part of our investigation we are deposing them on the e-mails."

Andre Pineda, the deputy corporations commissioner, told the Los Angeles Times that an obstruction of justice case against Deutsche Bank was a "possibility".

"We definitely did not get a satisfactory explanation from the [Deutsche Bank] IT folks, so we are looking to depose the outside counsel," he added.

The affair has led to the postponement of a settlement between Deutsche Bank and the SEC that is understood to have involved a payment of $80m (£49m).

Ten banks have so far agreed to pay out a total of $1.4bn (£855m) in order to settle the case.

In a statement, Deutsche Bank said it had failed to produce all the e-mails "inadvertently".

The controversy further highlights the increased scrutiny US law firms are coming under as the US' regulatory authorities move to restore investor confidence following the collapse of Enron.

Earlier this month the American Bar Association pledged to overturn its own ban on lawyers whistle-blowing on clients as part of a package of post-Enron reforms to its professional conduct rules.

Meanwhile, the SEC is currently in talks over whether to go ahead with proposals forcing lawyers to notify the regulator of suspected breaches of securities law.