Sullivan & Cromwell continued to chip away at BP plc’s potential liability to investors this week, scuttling the larger of two proposed classes in securities litigation related to the Gulf Coast oil spill.

In a lengthy decision issued on Tuesday, U.S. District Judge Keith Ellison in Houston refused to certify a class of BP shareholders who say the company duped them about its safety record for more than two years leading up to the Deepwater Horizon disaster in April 2010. The judge ruled that plaintiffs counsel at Berman DeValerio and Cohen Milstein Sellers & Toll were unable to show that damages could be calculated on a classwide basis for the group, the so-called “pre-disaster” subclass.