BP plc will face a maximum fine of $13.7 billion under the Clean Water Act for its Gulf of Mexico oil spill, several billion dollars less than feared, after a judge ruled that it was smaller than the US government claimed.

The ruling by federal magistrate Carl Barbier put the size of the worst offshore spill in US history in 2010 at 3.19 million barrels.

That was well below the government’s estimate of 4.09 million barrels, which could have led to penalties of up to $17.6 billion.

BP’s stock rose 2.4 per cent in London by 0920 GMT yesterday, outperforming the broader energy index, as investors worried about the size of potential penalties breathed a sigh of relief.

“The ruling is a step in the right direction of what appears set to be a long and hard-fought legal battle,” Barclays said in a research note.

Under a “gross negligence” ruling Barbier issued in September, BP could be fined a statutory limit of up to $4,300 for each barrel spilled, though he has authority to assign lower penalties.

A simple “negligence” ruling, which BP sought, caps the maximum fine at $1,100 per barrel.

The Clean Water Act penalties would come on top of more than $42 billion the oil major has set aside or spent for clean-up, compensation and fines.

About 810,000 barrels were collected during clean-up.