The preliminary ruling by the Delaware Court of Chancery, a premiere venue for shareholder lawsuits, found Merrill Lynch could have aided the Zale board breach their duties to investors. It is the latest ruling by the court that potentially exposes a Wall Street bank to damages in a merger deal.

Zale agreed in 2014 to be acquired by rival Signet Jewelers for $21 per share, or $690 million. TIG Advisors, which held nearly 10 percent of Zale stock, called the deal grossly unfair and shareholders only narrowly approved the sale.

Soon after the deal was announced, shareholders filed a class action challenging the deal price and named as defendants Zale's board, Signet and Merrill, a unit of Bank of America (>> Bank of America Corp).

The Thursday ruling by Vice Chancellor Donald Parsons dismissed the Zale board and Signet as defendants.

However, Parsons found Merrill could be liable to shareholders because it failed to disclose potential conflicts. Merrill Lynch never told Zale's board that a month before it was hired by the board the bank made a presentation to Signet's chief financial officer about acquiring Zales for $17 to $21 a share.

"I find it reasonably conceivable that this undisclosed conflict hampered the ability of Merrill Lynch and, consequently, the board to seek a higher price for Zale's stockholders," wrote Parsons in a 59-page opinion.

A spokesman for Merrill Lynch declined to comment.

Last year, shareholders of ambulance company Rural/Metro won $76 million in damages against RBC Capital Markets, a unit of Royal Bank of Canada (>> Royal Bank of Canada). Like Merrill with Zale, RBC was alleged to have aided the Rural/Metro board breach its duty to shareholders.

The Delaware Supreme Court heard oral arguments this week by RBC seeking to overturn that ruling, which has prompted many shareholders to expand their class actions beyond board members to include deep-pocketed investment banks as defendants.

In addition to the class action, some Zale investors are seeking appraisal of their stock. In an appraisal case, shareholders who are forced to surrender their stock in a buyout can ask a Delaware judge to determine the fair price for their shares.

(Reporting by Tom Hals in Wilmington, Delaware; Editing by Cynthia Osterman)

By Tom Hals