BP stock hit by court negligence ruling that could cost oil company another $18 billion

BP shares slumped 5 percent after a judge ruled that the oil giants's reckless conduct led to the worst U.S. offshore oil spill, a decision that could cost BP another $17.6 billion.

BP shares fell $2.72, or 5.7 percent, to $44.99 around midday, reducing the company's market value by $7 billion. The shares were near $60 per share just prior to the April 2010 spill.

The ruling means BP now faces a fine of $4,300 per barrel of oil spilled. The number of barrels spilled is being debated but is likely to fall between 2.4 million and 4.1 million barrels, which makes for a fine of between $10.3 billion and $17.6 billion.

BP, which has already paid billions to those hurt by the spill, says it will appeal the ruling.