Big blue-chip law firms are losing potentially lucrative assignments to smaller firms even as the industry sees a spike in lawsuits against banks stemming from the financial crisis.

The reason for the change: ethics rules that govern conflicts of interest for lawyers and their firms.

Law firms usually can't sue or investigate banks that they have represented, unless the clients take the unusual step of waiving the conflict. Thus, many small to midsize firms, which count fewer banks as defense clients, are filling a growing demand for conflict-free lawyers able to file lawsuits against banks.

Litigation against banks includes claims that they misstated the value of mortgage-related securities or reneged on financing agreements. Ambac Assurance Corp. recently sued a unit of Credit Suisse Group, alleging the securities firm made misleading representations about attributes of home-equity lines of credit backing bonds the insurer guaranteed. A Credit Suisse spokesman says the suit lacks merit. Last year, MBIA Insurance Corp. sued Countrywide Financial Corp., now owned by Bank of America Corp, claiming it misrepresented the quality of mortgage-backed securities. Bank of America declined to comment.

Consolidation in the banking business has made it only harder for law firms to handle lawsuits against banks. It is increasingly difficult, lawyers said, for firms to find a major bank they haven't represented at some point. As a result, they are bumping up against the conflict-of-interest rules formulated by the American Bar Association and state bar groups.