According to the Irish bank's complaint, filed in London's High Court, Mirabaud Securities suggested (falsely) that the US firm had withdrawn a $2 billion credit line.
The suit was filed following a call by the UK financial regulator for people to submit cases of traders deliberately manipulating share prices to be submitted to them.
Price fluctuations for the shares of the nation's biggest lender, HBOS, had led the Financial Services Authority to make the call - as these price movements were apparently caused by rumors and speculation.
Contained in the complaint was a transcript of an email sent by a Mirabaud worker on February 29th, which stated: "Anglo-Irish, ML pull a $2 billion credit line? Rumor.''
Anglo Irish terms this message "false and defamatory".
Mirabaud, owned by a private Swiss banking firm, has thus far refused to comment on the case, Bloomberg reports.