A US judge has upheld the ruling that BP's conduct around the 2010 Gulf of Mexico oil spill was "grossly negligent".

Judge Carl Barbier in New Orleans is weighing up how BP should be punished for the disaster and the oil giant had appealed to him to overturn his initial verdict, which was refused. BP could now face $1bn in penalties for contravening the Clean Water Act, Reuters reports.

Barbier had initially ruled that BP had behaved with gross negligence on 4 September. The trial is expected to resume in January 2015.

The judge wrote: "BP's assertions that it was 'unfairly surprised' and 'prejudiced' by the court's reliance on this testimony lack any basis in fact or law. Rather, it seems BP was a 'victim' of its own trial strategy."

The disaster happened on 20 April 2010 during the final stages of exploratory drilling on a well at the Deepwater Horizon rig. An explosion on the rig killed 11 people, with the resultant oil spill continuing until 15 July.

BP has already agreed to pay $2.4bn for violating the Clean Water Act, with Barbier describing the company's actions as "reckless". He attributed 67% of the blame to BP, 30% to Transocean – the rig owner – and 3% to Halliburton, which had carried out cement-sealing work.