The boardroom war at pubs group Mitchells & Butlers took a new twist todaywhen a powerful shareholder group issued a warning to investors. The Association of British Insurers had made a so-called "amber-top" warning to M&B shareholders, alerting them to think carefully about how to cast their votes ahead of next week's annual meeting. Today however, the ABI changed that to a red-top – indicating that serious corporate governance issues connected to an alleged backdoor attempt to gain control of the business were at stake. M&B, which is due to hold its annual meeting in Birmingham next week, is locked in a feud with its biggest investor, the billionaire Joe Lewis, whose Piedmont investment vehicle owns nearly 23% of the pub group's shares.