The National Credit Union Administration filed a federal lawsuit in New York in 2013 on behalf of two defunct credit unions, Southwest Corporate and Members United Corporate.
A spokesman for RBS did not immediately respond to a request for comment outside regular business hours.
The NCUA, which regulates, charters and supervises federal credit unions, has brought several lawsuits against various banking defendants over securities sold to several credit unions that failed during the financial crisis.
The agency previously recovered more than $1.7 billion from several banks, including JPMorgan Chase & Co, which agreed to pay $1.4 billion in November 2013; Bank of America Corp, Deutsche Bank AG; Citigroup Inc; and HSBC Holdings Plc.
In the RBS case, the NCUA claimed the bank systematically ignored underwriting guidelines for the securities, which were rated triple-A at the time they were purchased by the credit unions for a total of more than $300 million in 2006 and 2007.
The credit unions were placed into conservatorship in September 2010, according to the lawsuit.
The case is National Credit Union Administration Board v. RBS Securities Inc et al., U.S. District Court for the Southern District of New York, No. 13-6726.
(Reporting by Joseph Ax; Additional reporting by Nate Raymond; Editing by Steve Orlofsky and Tom Brown)
By Joseph Ax