Nomura Holdings Inc. and Royal Bank of Scotland Group Plc. may face $500 million in damages for what a judge called an "enormous" deception in the sale of defective mortgage-backed securities, a ruling that may spur other banks to settle similar claims tied to the 2008 financial crisis.

Nomura and RBS were excoriated in a 361-page opinion by U.S. District Judge Denise Cote in Manhattan, whose ruling followed the first trial of claims that banks sold flawed securities to government-owned mortgage companies. After a three-week trial, Cote said they misled Fannie Mae and Freddie Mac and set a damages formula that may result in the government winning about half its original claim of $1 billion.

"The offering documents did not correctly describe the mortgage loans," Cote, who heard the case without a jury, wrote Monday. "The magnitude of falsity, conservatively measured, is enormous."