If the U.S. court makes the maximum award possible, the oil giant could be on the hook for $17.6 billion in fines under the Clean Water Act.
This contrasts with the $3.5 billion BP said they had put aside to settle claims in their last annual report. The final amount will not be announced until early next year, and is then subject to appeal – and BP has been very firm in its commitment to appealing the judgment, and the figure put on the size of the spill.
The Citi upgrade also comes at a time when the oil giant is gearing itself up for further damage from the West's sanctions against Russia over the tensions in Ukraine. BP has a long – and sometimes turbulent – history in Russia with substantial holdings in oil and gas exploration.
One thing which has kept BP investors loyal through the lean years is its regular dividend payouts, which were only suspended during the darkest days of 2010.
These are unlikely to be affected for some time, if at all, by the most recent judgement, according to many analysts.
Jason Gammel, an equity analyst at investment house Jefferies, said in a note that the payout should not be hit because of the oil giant's good liquidity position.
"There's still plenty of arguments to be heard...and plenty more ink to be spilled about this claim," Jim Nicholson, senior vice president, Asia at Argus Media, told CNBC.
- By CNBC's Catherine Boyle. Twitter: @cboylecnbc