(Adds quote from court)
HOUSTON, Sept 4 A federal judge has found oil giant BP Plc "grossly negligent" for its role in the 2010 spill in the Gulf of Mexico, a ruling that could add billions of dollars in fines to the more than $42 billion in charges taken so far for the worst offshore disaster in U.S. history.
Shares of BP traded in the United States fell 5 percent, or $2.40, to $45.31.
"The Court concludes that the discharge of oil 'was the result of gross negligence or willful misconduct' by BP, the ruling from U.S. District Judge Carl Barbier said.
The company has shrunk since it was forced to sell assets to pay for the disaster aboard the Deepwater Horizon drilling rig that killed 11 workers.
Barbier has yet to assign damages from the spill under the federal Clean Water Act, though previous calculations by Reuters have shown fines could run to $17.6 billion in the costliest scenario.
Barbier apportioned 67 percent of the fault to BP, 30 percent to Transocean Ltd, which owned the drillship, and 3 percent to Halliburton, which did cement work on the Macondo well that blew out. (Reporting by Sudip Kar-Gupta and Karolin Schaps; editing by Kate Holton; Writing by Terry Wade)