An attorney who helped a company secure a trademark on Chalk Paint must face a malpractice lawsuit alleging he switched sides to challenge the same mark as generic.

U.K.-based paint company Annie Sloan Interiors Ltd. can proceed with claims alleging attorney Timothy R.W. Kappel breached fiduciary duty and committed legal malpractice, the U.S. District Court for the Eastern District of Louisiana ruled.

Kappel represented both Sloan and Sloan’s U.S. distributor Jolie Design & Decor LCC, before dropping Sloan and later representing newly-created affiliate Jolie Home LLC in a pending challenge at the U.S. Patent and Trademark Office.

The June 14...