Bernard Madoff, the founder of Bernard Madoff Investment Securities and a former chairman of the Nasdaq stock market, was on Thursday charged by federal prosecutors with a multibillion-dollar securities fraud.

Mr Madoff, 70, told employees that he estimated the losses from this fraud to be “at least approximately $50bn”, according to federal prosecutors. Mr Madoff’s firm had about $17bn in assets under management in January 2008, according to regulatory filings, and officials close to the investigation said they did not know whether the $50bn estimate was correct.