Credit rating downgrades of troubled bond insurers could trigger a potential financial “tsunami” that could be as far-reaching as the subprime mortgage crisis, the chief executive of Deutsche Bank warned on Thursday.

The alert by Josef Ackermann came as the banking sector continued to suffer from fears that rating downgrades to bond insurers, or monolines could lead to another round of writedowns of investments and renewed capital constraints. If the monolines are downgraded, the bonds they insure fall in value.