Investors suing Washington Mutual Inc., the former owner of the biggest U.S. bank to fail, won certification as a class-action case of their suit alleging shoddy lending practices.

Shareholders who lost money on stock purchased from October 2005 to July 2008 can proceed with claims under a single lawsuit, U.S. District Judge Marsha Pechman in Seattle ruled Tuesday, according to court documents. The judge appointed the New York-based law firm Bernstein Litowitz Berger & Grossmann to lead the plaintiffs' case.

The lawsuit consolidates more than 20 cases filed against Washington Mutual that claim the bank secretly lowered lending standards, artificially inflated home-price appraisals and failed to disclose its deteriorating financial condition when the loans began to fail.

John Wolfe, an attorney representing Washington Mutual defendants, didn't immediately return a voice-mail message seeking comment.

The named plaintiffs in the case include Ontario Teachers' Pension Plan Board, the largest single-profession pension plan in Canada, and four other pension groups, according to court documents.

They seek to represent tens of thousands of shareholders who lost money on three types of preferred stock purchased between October 2005 and July 2008 and certain securities offered by the bank in 2006 and 2007.

The shareholders argued the case should be granted class-action status because their claims are typical of what other investors experienced and are based on common legal issues.

WaMu filed for bankruptcy Sept. 26, 2008, the day after its banking unit was taken over by regulators and sold to JPMorgan Chase for $1.9 billion. Before it failed, Washington Mutual Bank had more than 2,200 branches and $188 billion in deposits.