The ruling, which is being contested by the company, paves the way for billions of dollars in additional civil penalties under the Clean Water Act.
Louisiana district judge Carl Barbier, who held a trial without a jury last year, said BP bore 67 per cent of the blame for the Macondo well disaster, which killed 11 workers in April 2010.
The ruling concluded the discharge of oil "was the result of gross negligence or wilful misconduct" by BP.
The oil firm, whose shares fell six per cent following the publication of the ruling, said it would immediately appeal to the US Court of Appeals for the Fifth Circuit.
It said: "BP believes that the finding that it was grossly negligent with respect to the accident and that its activities at the Macondo well amounted to wilful misconduct is not supported by the evidence at trial.
The law is clear that proving gross negligence is a very high bar that was not met in this case."